Heah Planetbuster,
welcome to the wonderful world of PBEM SMAC! If you've never played before and are just online for the first time, you will want to download the SMAC version 4.0 patch from the official Firaxis SMAC website:
http://www.firaxis.com/smac/
Once you have done this, you will be able to play SMAC PBEM. Also, the following is a Civgaming thread with a lot of links to other SMAC(X) sites, including Dreifels' site where he has done a screen capture for each step of playing a PBEM turn:
http://www.civgaming.net/forums/show...=&threadid=888
Civgaming is also another good site for playing PBEM's as well. Here is the game signup thread (and just like Apolyton you will have to register before you can post to a thread at Civgaming):
http://www.civgaming.net/forums/show...=&threadid=923
Finally, to answer your question regarding commlinks/ diplomatic screens: this is also turn based. So when you open your PBEM turn one of the other items you will do besides base maintenance and moving the units around is that if you receive any diplomatic correspondence from a human player, you then respond to it at that time, or if you want to initiate correspondence with another human player you do it at this time as well: you then have to wait until your next turn for a reply/ confirmation of any diplomatic messages from human players(i.e. after the other human(s) has played their next turn). AI Diplomatic communication is still instantaneous. FYI.
Hope this helps, and again, welcome to SMAC(X) PBEM!
D