In AI. Creat path/mask, Copy it.
In AE. Creat a new Soild Layer (Ctrl Y). Paste into the solid layer.
You should have a mask in that solid layer the same as it was in AI.
If you have a problem, try tis from an earlier reply to a similar question:
If you go to prefereneces==>File Handling and Clipboard then check AICB. This should allow you to copy and paste a path from Illustrator CS into AE 6.5