Well I tried it again and I still didn't manage to log in... :/
I tried the mail and the password from the ispconfig user for that webmail site and that didn't work.
As Mailserver I have to enter the adress I typed to go on the webmail right? (mail.myserver.com)
I logued on the mysql database btw to see if I could use something there and the user table seems to be empty, is that normal?