Here is the instruction how to enable "atMail" in Plesk 9
1. Login at your server
2. Click on "Mail Server Settings" (under "Server")
3. At "Webmail", make sure atMail is checked (click OK to save)
You have to enable this for each client, or you can set atMail as default by editing the domain templates.
4. Select the domain and to details page for the domain
5. Click on "Mail accounts"
6. Click on "Mail settings"
7. Choose desired the webmail client and click OK to save.
That's it.
You can also set "atMail" as default webmail client in domain templates
1. Click on "Domains" in left menu
2. Click on "Domain Templates"
3. Edit exisiting template or create a new one
4. Choose desired the webmail client and click OK to save.
I hope this helps.