How do I set up a VPN on a DMZ'd iMac? I have an iMac set up as my server machine. On the network, it is in the DMZ. The rest of the computers in the network use an internal 10.0.0.x IP.

I have OS X Server's VPN set to give clients IPs in the internal network, and it seems to give the client the IP (connecting on my iPad and checking the connection status shows this), but I can't access anything on the local network or internet while connected. On a Windows 7 client, I get a "No Internet acccess" or "No network access" error.

I should also note that when I select IPs to give to clients in the VPN settings pane of Server, a slide-down dialog notifies me that there is an "Unknown Network In Range". I have the range set to 10.0.0.120 to 10.0.0.130.

EDIT: I should clarify why I have the machine DMZ'd. I have done this because Server wouldn't allow me to set up Open Directory without the host/domain name I had set linking to the iMac's IP, and because my router's port forwarding is rather awful. I should also note that I have enabled the firewall to keep things safe.
You have an iMac setup as a server, and it is DMZ'd?  Probably a very bad idea.  VPN provides protection.  With it in DMZ mode, all you need is Apple Remote Desktop and it's much faster.  Albeit, much less protected.  

I'd UN DMZ that machine, and setup a proper VPN.