This isn't exactly the proper forum be be posting this question in, but I'll do my best to help.
Webmail is defaultly on in most versions of PLESK. To access the webmail page you go to
http://webmail.domain.com and login with the user / pass you setup. In the newer ver's of PLESK you can limit webmail access for users, this would be done under the domain properties and then under the mail button. Select the user and then you can specify if they have webmail access or not.
Hope this helps.
Regards,
-S