3 Cheers and a Big THANKS to "Rosanne"
I have at iPage too and have a few add on domains. Configuring a domain in to domain mapping gave me "403 Forbidden Error". I stumbled on this post and as it is said, I went to the "Domain >Pointer" and chnaged it from the default home directory to the folder where the wordpress for the primary website is installed.
Guess What?
It worked for me too.
Thanks once again.