Enable File Sharing is not a function of Airport,

rather it is a function of OS X.
You will find it in System Preferences > Sharing.

what symptoms is Time Machine exhibiting?

Drive not mounting on the desktop?
Starts but never finishes?


we're going to need much more information on your machine and the problem before starting to look at this problem.
For a start, OS X version and what are you using as a Time Machine drive would be helpful