There's no PSN support in MultiMan since your PS3 already has native PSN support. You have to use ReActPSN with RAP files and the PKG of your desired game. ReACtPSN "Re-Arms" the PSN game with the RAP file to allow you to play it.
I'm not sure if I should tell you how to get RAP files since the program also lets you DL the PKGs and GBATemp has the WAREZ forum rule.