After Effects will only run on 64 bit systems. Which operating system are you using?
If you're running Win 7 32 bit on your machine then AE will not be available for download onto that machine.
Macs running OSX Lion and newer are OK since those operating systems are 64 bit only anyway.
Same goes for Premiere Pro and Encore.
Refer to the tech specs on the product page. As John said, all current Adobe video apps will only be available on supported 64bit operating systems...