Needs to be registered, been waiting 4 the activation screen to finish, been almost 3 days..

You can't. You have to have data to use this device on T-mobile network.

Let me start by saying I'm not an expert, but after going through a lot of gyrations I came up with a solution that worked for me. I'm sharing it here because I haven't seen a complete concise answer anywhere.
The activation problem with Sidekick Slides is common. Most of the time, people buy a Slide without realizing that you have to purchase a "sidekick plan" from T-Mobile to get it fully activated. These phones were build especially for T-Mobile, so they're designed so that you have to have a Sidekick rate plan first to get the phone activated. By the way, this applies to a new phone as well as a phone that's been activated before, but has been 'hard reset" (where you hold the R button while re-booting.. My advice, DON'T DO THIS! Because you have to re-activate the phone.
However, there's good news for all you non-T-mobile folks who bought a Slide on eBay (or from a friend etc.) only to find it needs activation. Here's the solution. If you search eBay for "sidekick activation" you can find folks selling prepaid activation kits for around $15. They include a sim card and about $10 worth of sidekick prepaid services. Buy the kit, install the sim, and go online to register your phone (registering by phone is a pain with T-Mobile). Online you have 3 plan choices: Pay By The Day, Pay As You Go, and Sidekick. Pick Sidekick, and then provide the requested registration information. When done, your phone should be ready to go and should activate within about 5 - 10 minutes. You'll be charged $1 a day and airtime for calls, so it's not cheap, but at least the phone won't go to the activation screen when you startup.
The next question is, how to unlock the phone for other networks? IF it's not unlocked you will be prompted for special code (this is different than the user-defined password that simply locks the phone against unauthorized users). It won't work on other networks until it's truly unlocked. Again, the trusty folks at eBay can help. I searched for Sidekick Slide Unlock Codes and found numerous vendors, ranging from $6 to $12, who provide permanent unlock codes for Slides. I found on that provides the code via email (others want to mail it to you) and got my code within 30 minutes. So now I can remove the T-mobile sim and put my ATT (or other) sim in and go. I should mention that some of the Sidekick features ONLY work if you have a sidekick plan, but that's T-Mobile's and Danger's devious attempt to make us miserable. Good Luck! Hope this helps! Rick

try pressing menu (the three bars) and L at the same time. i had an at&t sim card too and that worked.

Menu on it's own doesn't work - it will take you to the "emergency call" screen. You need to press "menu" + "L" to bypass the activation screen.

I have the same problem. But it says it could take up to 48 hours for the Welcome screen to appear. (Usually wouldn't take that long, usually it'll take about 1-3 hours)

If you bought it from a retailer the code is either all zero's, or the last 4 of the phone number. If you bought it off of eBay, go to T-Mobile and they'll do it for you

try turning it on and typing 1234 it should work it worked for me and if that doesnt work try goin to a cell phone repair shop they should be able to unlock it. if not get your money back for the phone and buy one at the tmobile store. some stuff on ebay doesnt work i bought an ipod off ebay and it broke the next day it broke so try gettin your money back if nutin else works!

FLIP THE PHONE OPEN AND PRESS AND HOLD THE MENU KEY WHILE PRESSING L ON YOUR KEYPAD THIS SHOULD BY PASS THAT SCREEN...ENJOY

press menu and the letter L at the same time so u dont have to wait 2 days of no phone using

hold down the menu key and press the L button