Forefront TMG translate private to public ip

I recently installed Forefront TMG as a 3-leg perimeter firewall. What I want to do is: I have a public network range (10.22.xxx.xx0 to 10.22.xxx.xx9), and some servers in the DMZ/perimeter network.

How can I configure that for examle the DNS server with IP 192.168.2.1 can be accessible from the internet through IP 10.22.xxx.xx1?

I'm struggeling with this for over a day now, all help is appreciated!