How can i make domain2.com the master domain.

Right now i have a domain as the master, so when i want to add scripts such as guestbooks...... it automatically goes to www.masterdomain.com/guestbook... when i want it to be www.addondomain.com/guestbook

Can someone either

A) help me switch domains to which is master

or

B) Show me how to make the addondomain able to make guestbooks after it.

Appreciate the help

- Bastard@BDCBastard.com