Newbie --->addon domain setup help!

I have setup and configured "STANDARD" and "MULTIPLE" installations but both only work from the the http://my.domain.com/public-html/addondomain.com/gallery2 link
and all I get from the http://addondomain.com/gallery2/main.php is a HTTP 500 error.

I guess I should say I only want the gallery to be attached to my second domain.(addon domain in cpanel)
I was also wanting to keep a html page and link out to the gallery. Hence the sub folder "gallery2" to keep things organized in my root.

I have read so much my eyes are bugging out.... I love to figure things out myself, but after days of trying I am finaly asking for help.

Thanks in advance