When you create the email account, make sure you set it up as Other (not as a .Mac or MobileMe account.) Then select IMAP as the type of account. Enter the name and auto-forwarded email address. (This is the email address that will show as the "From" email, even though you're using your MobileMe account.)
For your mail server settings, use your MobileMe settings:
- Incoming settings:
- Host Name: mail.mac.com
- User Name: MobileMe user name
- Password: MobileMe password
- Outgoing Settings:
- Host Name: smtp.mac.com
- User Name: MobileMe user name
- Password: MobileMe password
- Advanced:
- Use SSL - On
- Authentication - Password
- Server Port - 587

