Removal of Blade

I could not find instructions or video on your model, but on your model the brushes are under the plastic cover opposite from the blade.
if the cover is removed you should be able to see how to replace them.

Dull Blade most likely

Wrong Blade maybe

Wrong cutting depth setting on blade
or the way your using the saw

Hard to say without seeing it but....wrong sized blade, bent blade, clamp missing or installed wrong, bent guard, or bent blade shaft. I'd look in that order. Hope that helps.

ereplacementparts.com

There is a button on side that lock blade . Push on button turn blade by hand till you feel button engage now you can tighten bolt left hand threads

Should be a locking mechanism on back of saw push or pull the hold blade still...then use adjustable wrench to loosen nut on saw blade..pull off blade..slap a new one on..and away you go!!

The locking button on this saw is designed for safety. If you are attempting to change the blade it should NOT be plugged in. It sounds as if someone has pushed the lock button while the motor was running and sheared it off.
The only tool that is required to change the blade is the supplied wrench or an equivalent box end. If you cannot make the repair yourself for have the equipment repaired you can take a pair of vise grips. (Use cardboard to prevent warping or gouging the blade) and clamp the edge of the blade below the teeth. The vise grips will stop at the front of the saw as it is too large to pass by the guard allowing you to loosen the nut. Clamp the new blade in the same way only the back of the saw will be used to tighten.

I highly recommend that you either repair yourself or have the tool repaired by a professional.

Parts schematics are all over the net for this tool, here is an example:

http://www.toolpartsdirect.com/cgi-bin/schematic.cgi/skil/5150

As you can see most of the mechanical parts are only a few dollars and it could save you from serious injury during operation.

look for the button on the right side of the handle. Other models require you to put a phillips head screwdriver into a hole in the blade to hold it against the body of the saw.

It sounds like you bought a lemon possibly. I am thinking that the gears are bad. What you can do is remove the blade and unplug the saw. locate where the screws holding the bearing cover are and remove them to get to the gear and check it. Let me know if you need more help

First, there is no blade lock button on the Skil model 5150. I called the Skil customer service and they emailed me the exact manual for this circular saw. The method I used to loosen a VERY TIGHT blade stud was: spray with WD-40, imbed the blade in scrap wood, set Skil wrench (usually found in back of foot under handle) on so it could be struck going in counter-clockwise direction. Use hammer to strike handle of wrench a couple of times. To replace blade, finger-tighten blade stud, then use wrench to tighten 1/8 turn or (45°).