FAQ says:
"[...] a source port is just the executable code for the game, you still have to provide your own copy of the game's data.
So for example, using Quake III: Arena again, the game consists of both an executable and a set of data files (pk3 files, in this case). The download for ioquake3 is an executable that is up to date and has been maintained to work on modern Macs, but you still need to acquire the pk3 files from a legal source, such as an existing installation of the game from disc or Steam or GOG."