Hi my forum is located at mydomain/forum but do i increase that links popularity or the main url mydomain any help appreciated.
You need to do both. Increase link popularity for ALL your pages (where and when possible). So if you ant links to "your forum" it should point to domain.com/forum and then "your site" would be domain.com
But which will increase my forum rankings if i have alot of backlink to my domain will this also work to increase my forum as the index page does forward to my forum.
You may want to build your forum and have more posts. Even if you do get traffic, the fact that your forum is pretty empty will turn away your visitors. Paid postings might be the way to go to increase your post count.
the homepage will forward pr to the forum but in small "quantity", as for all other site pages whick are linked in the homepage... so do the math
A good question might be ... which of the pages (if any) have you optimized for the search engines (ie. meta tags, keywords, ect.)
i would build for all pages throughout your site. as has been suggested above it would be a good idea to build your forum up as well with more posts etc
It's a matter of keywords. If your main page is optimized for they keyword - car replacement parts and your forum is car designer forum, then you have to go get the right anchor links back to your site. You have to choose what kind of keywords you're going for THEN start your linking strategy.