You have to manually "rate" each individual post. I have over 50 students, who all post thoughtful stuff, but it is the few who occasionally forget to post at all one week.
I know I can review their activity reports, but then what's the point of even having a gradebook.
In last year's class, I didn't even use the gradebook at all, I don't see the use.
I have 180 students that I monitor and rate the posts. I find it easiest to rate them as I read them (if using a point system Moodle will not allow more points than what was assigned). You can set up the rating system for a numerical grade and do the sum of all posts, or simply a satisfactory scale. I find the system very easy to use.
What specifically are you trying to accomplish with your forums?
My instructions will work for the new Moodle gradebook (Moodle 1.9) where you can create a graded item entry independent of a Moodle activity.
To accomplish what you wanted to do (kind of like a completion grade for the forum) I would select in the Forum settings to have "no grade" (since it's not capable of just being a completion grade). That way it doesn't show up in the gradebook.
I would put in a graded item in the gradebook named the same as the Forum (that way students think it is the same thing) and make it worth however many points you want.
You can then view the activity reports for the forum and just add the appropriate amount of points to each student that appears as having posted to the forum.
Here's a link to a brief screencast that illustrates my example:
Very helpful and great screen video. thank you