How do I replace the drive belt on a scag liberty z zero turn mower? Belt seems to be okay, just came off while mowing for some reason and now won't move. Mower part works fine.

usually if the pins are pulled out that is the freewheel position. push them in to drive. if the trans locks in drive and it is hard to push it should be ready to mow. unless the drive belt is stretched or broken or a linkage is out of adjustment or broken.

Safety and Preparation Serious injury can occur if the engine or mower blades accidentally start while you are working with lawn tractors. To prevent this, drive the Murray lawn tractor onto a flat work surface and disengage the mowing deck cutting blades. Engage the parking brake to keep the mower from moving; chock the wheels with a wooden block or brick. Adjust the mowing deck to the lowest position. Turn off the engine and remove the key from the ignition. While not necessary, you may even wish to disconnect the spark plug wire from the spark plug. Mowing Deck Removal You must remove the mowing deck on a Murray lawn tractor to gain access to the belts. Remove the adjustment rod hairpins, followed by the adjustment rod pins from the front left and right sides of the mowing deck. At the rear on the left and right sides, pull out the retaining hairpins and slide the mowing deck suspension links from the adjuster plates to separate the rear mowing deck from the frame. Remove the blade control rod stud, including the hairpin and washer, from the top of the blade control lever shift linkage at the left rear of the deck. Remove the front hanger rod, including the hairpin and flat washer that hold the rod in place. Loosen the belt guide retaining bolts with a socket wrench and remove the engine stack pulley belt so you can slide the mower deck out from under the tractor Transmission Drive Belt Installation Slide the new transmission drive belt between the engine stack pulley belt guides, over the belt guide plate and under the engine stack pulley. Fit the transmission drive belt around the top engine stack pulley. Push the free end of the transmission drive belt up through the transmission shifter hole in the lawn tractor floorboard. Slip the transmission drive belt loop over the transmission shifter. Pull the transmission drive belt down through the transmission shifter hole. Fit the transmission drive belt around the transmission drive pulley and tighten the transmission drive pulley belt guide retaining bolts to hold it in place. Fit the transmission drive belt flat side around the idler pulley. Slide the idler pulley onto the idler pulley stud while holding the belt in place. Tighten the transmission idler pulley retaining nut with a socket wrench to secure the idler pulley to the idler pulley stud. Mowing Deck Drive Belt Installation Pull back on the right side mowing deck mandrel pulley spring-tensioned belt retainer. Guide the new mowing deck drive belt around the right side mandrel pulley and release the belt retainer. Pull the drive belt spring-tensioned retainer away from the mowing deck idler pulley, guiding the deck drive belt around the idler pulley. Fit the mowing deck drive belt around the left side mandrel pulley and under the rear mowing deck cross link rod between the right and left side mandrel pulley. Slide the mowing deck under the right side lawn tractor frame, reconnecting the deck to the frame in reverse order of removal. Engage and disengage the mower deck while driving the mower to test the belt for proper operation.

Sounds like you have 2 separate problems.
Once you get more gas and get the engine running you will have solved the first one.
If the blades will still not engage, shut the engine OFF and check the drive belts, idlers, and spring on the tension idler.

6 hours and it is probably still in warranty. Consider taking it back.

It does sound like a belt problem, if possible replace it with a kevlar belt, they last way longer and will survive more abuse.

http://www.scag.com/OpManuals/SVR/2014SVRIPL/SVR_IPL_Book_06271.pdf

The idler arm assembly

tighten or replace drive belt

Could be the drive belt is slipping. This is the belt that goes from the engine to the transmission. If it is worn it can slip. You can go to this link and view parts.
http://www.ereplacementparts.com/murray-46581x92a-2000-4634-lawn-tractor-parts-c-17887_17888_28185.html

It refers to the belt as ITEM 15. Belt, motion drive

First, wedge something in front of the front wheels (when you lift the back, there's no brake action). Get the jack from your car and position it to lift the rear of the mower chassis. Jack it up then put blocks under the rear wheels - you may be able to do both at once, or you may have to alternate - then lower onto the blocks. Get enough space. You'll have to take off the mower belt and remove the PTO clutch anchor (two bolts) and may have to take off the wires connected to the PTO clutch (connector may be above the chassis). Then pull off the spring that tightens the idler arm, remove the old belt (if it's still there) and install the new one. Refasten the spring to tighten the idler, replace the anchor and mower belt, and mow away. Good luck! Please rate my answer if helpful, thanks.