Ok ,
First you need to download a theme that is non ATT.
try the first post on this thread.
The download BB maps from RIM
BlackBerry - PDA GPS - BlackBerry Maps at BlackBerry.com
Then go to the site from the BB browser.
mobileapps.blackberry.com/devicesoftware/entry.do?code=maps_sbi
The BB maps icon will be in the applications folder of the new theme.
IT will NOT be in any ATT theme.
Both links work better from the BB but the maps app can be downloaded via your computer with the BB connected. It will not work (download) without it seen by Rim.
The second link will download an Icon to allow it to display on the NON att theme.