WordPress MU

when someone tries to sign up they receive activation email but if they click on the link, they get this error:

Not Found the page

sample

http://bledar.mastoraspc.gr/wp-activate.php?key=3fb3b31beb5ba37e

or

Username: alb
Password: 84ab6b8a
Login Here: http://alb.mastoraspc.gr/wp-login.php

try to create blog and you see

I replied in your other post. You need to enable DNS wildcards on your domain.