Yes, people still play multiplayer. I assume you've already installed COD2, so all you have to do is launch the COD2 Multiplayer application and join a server. Here's a tip: try to find a server to play in that has both low ping (below 100 at least, the lower the better) and no mods. There's a "Filter Servers" option that you can click that will disable servers with mods from showing up in the server list. Modded servers can be fun, but if there are a lot of custom maps on the server that you're attempting to connect to, you'll have to download them before you can play on that server. Since downloading files through COD2 goes at the rate of about 12 KB/s, it'll take awhile even to download a 5 MB file.
Hope this helped... PM me if you have any other questions. I've been playing this game for Mac since 2007. I could even make a video demonstrating how to get started if you want.