First, make sure there isn't an IT policy on the device, use the link above. A device that old probably has a policy on it, as most consumer blackberry owners most likely have upgraded by now.
Be sure you have the right OS for the right device. For Cingular (ATT) go here: BlackBerry - BlackBerry Desktop and Device Software Download Sites
Then, click AT&T then click in the drop down menu, 7290 Wireless Handheld. Download the one for English (if that is the language you want on your BB.
Make sure you have Desktop Manager installed. Next, download JL_CMDER from here: http://www.blackberryforums.com/gene...-jl_cmder.html
Hook up the blackberry to your computer, run JL and wipe handheld. The phone should now show an error 507 on the screen after the wipe. This is good.
Install the OS software. After you install the OS software, plug the BB into the computer and open DM. If the PIN number doesn't automatically detect, then click "None" and select the communications port that has your PIN on it (the PIN is going to be an alphanumeric pattern, mine is in my profile, for reference). When the device is detected, go to Application loader, and click Add/Remove applications, and if that doesn't do anything, try Update Applications. This is with DM 4.5, and I haven't done a wipe in a while, so I cannot recall which one actually loads the OS on to the BB.
Hope that helps. Just don't give up. It's not terribly difficut!