It sounds like something is going on with your clipboard. Are you on Win or OSX X? There used to be an issue with After Effects taking over the clipboard and not allowing paste, but this is largely resolved in CS6.
One thing to try:
1. Save your mocha camera export as a file.
2. Quit mocha
3. Make sure your copy/paste is working in AE
4. open up the saved file in a text editor, then copy to clipboard
5. try to edit>paste mocha camera
The only way to get mocha camera solve in to AE is through the plug-in and it seems that AE clipboard is not working right. You could also try removing and re-adding the plug-in.
Lastly contact Imagineer Systems support for more help.
Best,
Ross
Ross Shain
Imagineer Systems
http://www.imagineersystems.com