In Mail, click Preferences... Click on Accounts, click on the + sign at the bottom. Enter your details, Mail will recognise the @me account and set up all the rest.
If you are still having problems with Mail after this, you'll probably have to go through Apple Support pages eg.
http://support.apple.com/kb/HT2296
Sometimes you may have to check the ports aren't blocked/slow due to your router or ISP. You can get information on the ports Mobile Me uses from something like:
https://support.apple.com/kb/HT1625
FWIW.
I take it you're using iOS 5 beta?