You have to go to "C:\Users\YOURWINDOWSUSERNAME\AppData\Roaming\Raptr\data" open and edit the "game_detection_pc.json" Simply search for "RADS\\solutions\\lol_game_client_sln\\releases\\*\\deploy\\League of Legends.exe" and edit it to "League of Legends.exe". Save and set the "game_detection_pc.json" as read only. Restart the raptr Client and try again to manually add the game.
Sometimes the game detection may fucked up or the game disappears. Say thanks to raptr, who do not like if someone try to manipulate the game detection to work with modified games or games that have changed the game exe. (dota 2 reborn for example).