Thanks LadyEye... Yes my add-on domain is purchased.... I know I need to change the htm to html and I think my problems lie within my file management.
Maybe this will help you.... When looking at my File Url in File Manager my add-on url reads
www.mymaindomainname.com/my add-on domain name/index.html
so the thank you page follows suit
www.mymaindomainname.com/my add-on domain name/thankyou.html

I think I really have something out of wack here. This is my first time trying to implement an add-on so I dearly thank you for your patience..
I can provide you with as much info as you need if this isn't enough....
THANKS AGAIN!!!
Brian (committed newbie 4 life)