To activate your webmail in Plesk (9.2.x), login to your Plesk account with your user details given to you when you purchased your hosting account.

Select the "Mail Accounts" icon under the "Mail" subheading.

Select the "Mail Settings" icon.

Under the preferences subheading, select the webmail you would like to use from the drop down list.

In this case, either Horde, AtMail or None is possible.

What if: There is no option to select webmail or it is greyed out? 
The server may have the webmail option switched off (ask your admin).