It could be in a number of places, depending on your operating system.
Windows: Either C:\Program Files\AssaultCube_v1.1.0.4\packages\maps\mapname.cgz
or
My Documents\AssaultCube_v1.1.0.4\packages\maps\mapname.cgz
Linux: ~\.assaultcube\packages\maps\mapname.cgz
(I'm not sure at all about the Linux path.)
Mac is possibly the same as Linux
Windows: Either C:\Program Files\AssaultCube_v1.1.0.4\packages\maps\mapname.cgz
or
My Documents\AssaultCube_v1.1.0.4\packages\maps\mapname.cgz
Linux: ~\.assaultcube\packages\maps\mapname.cgz
(I'm not sure at all about the Linux path.)
Mac is possibly the same as Linux