Mac versions of JKA are only compatible on Mac versions prior to macOS 10.15 Catalina due to Apple dropping 32bit support for apps. This applies to Mac version of OpenJK and JKJALaunch.
If you downloaded Jedi Academy from Steam or the Mac App Store, chances are you can't join any servers because of this:
The only way to get around this is to do one of the two following things.
This is a new launcher app that allows you to join servers. It also supports command line options, and the option to specify a custom master server.
Download from the above link, and put it in your Applications folder, or anywhere that's convenient. Hold shift and double click it. You'll get a window like this:
Locate your JKA app. If it's the Steam version, it should be in the following directory:
Macintosh HD/Users/you/Library/Application Support/Steam/SteamApps/common/Jedi Academy/JKJA.app
If it's the app store version, it should be in your Applications folder:
Click okay and then choose Multiplayer.
Join a server and all should be well!
The other good option is to use OpenJK, an open source project that contains lots of bug fixes, and allows you to connect to servers on Mac. Click the above link and follow that tutorial to install it.