Some would complain creating mail user with control panel access in Plesk 10 is complicated and requires double work. Generally this would be quite easy action if the one proceeds with creating a user, rather than mailbox:
This way the one would get a mailbox and control panel user with aligned login and password. And no double-work is involved.
While availability of opposite direction (create User for Mailbox) would make it definitely more convenient, for most practical needs this solution should be sufficient. The key to understanding this approach would be the fact that Plesk 10 makes stricter separation between Mailbox (POP3/SMTP/IMAP access) and User (Control Panel access).
UPD: An easier solution of mail user problem is suggested in Plesk 11.5