This level, q2bball5.bsp, is for the Quake 2 Basketball Mod.

You should have extracted map5.zip in you quake2\bball\ directory.
The new server.cfg, weapons.cfg, and bots.cfg files should be in
your quake2\bball\ directory, and q2bball5.bsp should be in you
quake2\bball\maps\ directory. Just make sure you are preserving
the path when unzipping and you should have no problems.

q2bball5 has been added to the default map rotation by adding it
in the sv_maplist command in the different .cfg files.

If you just want to check out q2bball5 type "map q2bball5" in the
console.