I use Easynews.com.....since it is web based you do not a client that works on MAC. You could also use giganews as well. Same type of situation.
Both sites have a small monthly fee...usually priced by GB.
A few pages pages there were a couple of posts that had links for people new to news groups and I think it had a list of the different places to sign up as well as other instructions.
I have been using easynews for several years now and the only problem I have had with them is recently......they do not show incomplete files anymore....and the first posting of the NTSC version had some incompletes and it was hard for me to get those few so I could repair them. I ended up using a client to connect to easynews as they had the files.....just do not show them in their web interface.
Hope this at least gets you going