Internet Sharing won't show under Share settings on OS X Server 10.6.8
The server OS doesn't offer the one-click Internet Sharing.  It can be done, although the server OS wants to be much more involved than the client OS.


Go to your Server Admin.  
Select your server and click NAT (if you don't see it, click your server name, then click the Services tab at the right, then put a check-mark at NAT.  Now, back at the left hand panel, click the NAT service you just enabled.
On the Settings tab, make sure IP Forwarding and Network Address Translation (NAT) is selected, and choose the proper External network interface.
On the Overview tab, click the Gateway Setup Assistant... button.


At this point, unfortunately, I can't provide the details.  My 10.6.8 mini server is a DHCP client, so I can't continue with the assistant without altering my setup :(  

Do reply though with the results of the Gateway Setup Assistant...