Not everyone has htdocs it depends, usually I see htdocs on parallels plesk installations and public_html with whm/cpanel setups.
Try:
AdminCP > Maintenance > View PHP Info > *Scroll to find (or use "find" in browser options) the text "_SERVER["DOCUMENT_ROOT"]" and that will have something similar to: /home/accountname/public_html
So with that being said if the mod file was in lets say for example you use your forum in a folder such as forum it would be: /home/accountname/public_html/forum/filename.php so in the settings you would key in for full path /home/accountname/public_html/forum/