Are you on Mac or Windows?
If it's Mac, 3D Assistants 1.1 only runs on PowerPC (ie, G4, G5, etc) and up to CS3, since After Effects CS4 is Intel-only (Mac Pro, Macbook Pro, etc)
You need 3D Assistants/Planespace 1.3 or later, which added support for Macs with Intel processors.
If you're on Windows, I am not aware of any changes that would prevent it from working, but I don't really know.
Does anyone know if its compatible with AE CS4?
1.1 was the last update released before CS3. Compatibility with CS3 and later versions requires at least 1.2 (which was a paid upgrade), ore Red Giant's Planespace flavor. So to cut a long story short: Your version will not work in CS4 and you will have to contact RG. As a minor, you will have to use the Serial Number Converter and download the 1.3 installer. Since you never did upgrade the DA version, you may however need to buy one, if your serial is not recognized.