Just follow instructions to download bb maps. Since you already have the app on your phone, it will only download some 'ribbon' file. Once you download, it won't hurt to reboot, and the icon will appear.
Posted from my CrackBerry at wapforums.crackberry.com
What I have found with my 8310, 9799, and now the 9800 is that as soon as AT&T sends new service books or if you reboot BBmaps will vanish again and you will have to install the ribbon file again to get the icon back.