"mg_smash_dev" includes spawns for both terrorists and counter terrorists
"mg_smash_dev_t" only includes terrorist spawns to prevent the round restarting when one team dies.

Installation:
Move both map files into this directory:
"C:\Program Files (x86)\Steam\steamapps\common\Counter-Strike Source\cstrike\maps"

If you have steam or CSS installed onto a different drive then it wont be on the C drive.