Open Apple Mail en choose “File” followed by “Add Account”.
In the following window you should give up a name for the account, accompagnied by the email address and password.
Now you have to enter the incoming mailserver; mail.nucleus.be. Also here you should give up a username and password. The username is the part before the @ in the email address.
The incoming security also works with a password.
As outgoing mailserver you should use the one of your provider. At the bottom of this page there is a list of related articles. There is also an article containing the SMTP servers of belgian internet providers.
Most internet providers don’t require authentication for outgoing mail.