Try with the Minimum,which when i did i got my ad on the DP tools Pages. Then Bid more to get in the Forum pages.
Just start your site targeted campaign and keep tweaking the CPM until you start to see impressions. Keep in mind CTR is critical for a CPM buy, so make sure you write a bunch a excellent ads so that the AdWords system can fine tune your best performing ads. This has worked well for me on another mega site's forums. I also kept adding new ads as I had new interesting content. This kept the CTR and net CPM attractive.
I tried to go over $6.00 CPM and never got it to show up (Back in November)!! Luckly it didn't, cause I still became King!!