forum got a low CTR but the hi traffic will compensate that. concentrate on getting more member/visitors you would be fine.
to me your forum looks a little confusing and there is a ton of white space there, signatures are huge people might not know what they want to do, maybe a little more uniformity in the layout. On the left side put some color there to separate it and reduce the signature size. Put a adsense 728x60 ad in between as the 2nd or last post in a color that matches the side. Right now, people don't know where to click on anything.
After doing a lot of research into this and even having google themselves send me placement suggestions I found the best combination is: For unregistered guests, directly under the nav bar. Two large rectangles side by side, one showing adsense, the other showing a CPM network image. For registered users, directly under the nav bar. One 768 wide leaderboard. Under posts didn't work, it just annoyed people. On around 40-50k impressions, I make on average $70-$80 a day from adsense, $15 from Tribal Fusion (the CPM). I also run Kontera, but it's not popular - brings in around $3 a day!