i cant play with my friend

how i put in the resumen i cant play with my friend , we are trying to play the game but the game say that i dont have friend buy i have him how friend and i cant add him again , so when we start a new game or try to put mods map and enter to the game we invite him in to the game and he cant enter , i dont know if i have to use hamachi or other aplications , i try to found something online about this problem but it is imposible so please i need help because i buy it for playing with him 

sorry for my english but i dont speak this language well and thanks you