The preferred download method is setting /cl_allowDownload 1 in console!
Open the console by pressing Shift+Esc! Enter the command above and reconnect by typing /reconnect
You should be all set! Have fun!

If you are having problems with automatic download (/cl_allowDownload 1) you can download the referenced paks directly from here!
Copy the files into your /fs_homepath dir!

Linux users should find it at /home/username/.openarena/CTF for ctf, and /home/username/.openarena/fm for deathmatch server.

Windows users, search for a folder something like this one for ctf:
C:\Users\User\AppData\Roaming\OpenArena\CTF,
and a folder like this one for deathmatch:
C:\Users\User\AppData\Roaming\OpenArena\fm

It is a good idea to delete everything you find in those folders before copying these downloaded files. Save a copy of your q3config.cfg and autoexec.cfg to a safe place if you want to keep your settings! If you still have problems connecting after completing the instructions, delete your q3config.cfg too because that file is the reason you can't connect in the majority of cases!
You can even delete the whole directory! Not just the CTF and fm dirs, but the upper one too! Upon a successful connect you will autodownload the pk3's needed!

Still having problems? Delete your whole /fs_homepath directory! That should fix it! (Practically deleting the rogue .pk3 - which one? all! - from your baseoa dir. You will redownload everything needed, do not worry!)

You can also try the solutions listed here! (external link)