It's easy to do.

Add a column called "rating" to the post table, and then to the forum table.

Modify your postbit template to include something like

Then modify postings.php to take that into account. Some SQL like "UPDATE post SET rating = rating + $value WHERE postid = $postid" will do the trick. (Also one for the thread table as well) Obviously check the $value var for illegal use (like someone modifying the URL to read "&value=9999" to stuff the ballot.

Also do some checking to make sure that the current $bbuserid is not the same as the post's in question (so as to not allow someone to rate their own).

Create another table called "Post_rating" or somesuch, with two elements:
"userid, postid"

That way you check to see if the user has already rated the post, if not insert the primary key set (userid, postid) from the given information to keep a record of it.

Other things you can do:
check to see if the post is available (not closed or whatnot)
check to see if the user has rights to it (so you can't rate something in a moderators-only forum)
etc.

Even though this is fairly easy -- it's a good primer for SQL and PHP so you should give it a shot -- if you have any problems let us (me? ) know and we can help you through it!

Note, that was not intended to be a slam against you -- but learning PHP now will give you great flexibility in the future with modification of VB -- and it means you don't have to rely on other people taking forever to come up with solutions!