You would have to create a new 3D object (in .obj format - follow the Element 3D tutorials for creation - C4D format would work as well). This would work almost like a billboard would - then you would take your video into a layer in your comp where the Element project lives, and make it a custom texture. Then apply that to the "billboard".
If you don't want the billboard to show as such, take the texture map from the building you want to use, open it in an AE comp (change the name of the texture, so you don't trash the original), then composite your video where you want it to sit on the building. I think the first method may well be the easiest, and depending on whether the video is a "hero shot" or just passing by in the shot, could work fine.
One step I neglected to mention, if you're doing the actual new .obj, is that you have to put that in place on the building, in 3D space. I assumed you'd know that, but not necessarily a valid assumption. I do 3D all the time, so I assumed it - shame on me...
Thanks for your help. I don't have or know how to use any 3D software like C4D. I wanted to place a person on a building that was shot on a green screen so I would need to be able to key them out. I was hoping for some sort of way to attach them to a null object or something so their position would be the same as one of the buildings. Is that even possible?
Just make the footage 3d. and use the front, left, top view to position it where you want. Both AE's 3d position and Element's are the same. Once alligned they will continue to be so. If you animate anything else besides a camera, like Element's particle origin. You'll have to create a null to the particle Group (verion 1.6+ has a feature to help you with that). Also if you need masking check out the tutorials on matting capabilities.
From what I've read and tried, Element 3D doesn't play too well with the 3D within AE, but, that said, it's worth a try. You might try using one of the built in basic objects which come with Element to see if you can do it that way. Take the keyed green screen shot, put it in a layer, use it as a custom texture, then position it in Element's 3D space. You know, I'm not entirely sure whether or not element will take a custom texture with an alpha...due to OpenGL. Take a look around here:
If you are using Element 3D V1.6 or above then you can use the generate 3d position to create a null attached and aligned to the surface of any 3d model you are using in Element. Then just copy the position and orientation to your video layer in 3d mode. Here is a tutorial: http://www.youtube.com/watch?v=dSN2OpbV18g#at=124