Is it a .pkg file or game backup?
If .pkg file just click on it in mM and it will begin the install.
If it's a game backup, make sure that;
a) the PARAM.SFO file is included and in the correct place..eg dev_hdd0>GAMES or GAMEZ>game title/ID>PS3_GAME>
b) the game is in the dev_hdd0>GAMES or GAMEZ>...folder
Refresh the list if necessary.