Make sure that the two notches are lined up on the motherboard and on the memory, take out the clip on the top, and then put a little pressure on the memory, it has to like snap in, and can sometimes be a bit tough, make sure you ground yourself so you dont fry nothin, and once it snaps in, Then put the clips on.
EDIT: the problem sounds like one end of the stick just isnt in the slot enough, just go ahead and put alittle pressure on the side that wont go in and it should pop in, even if the clip wont go on, your MB should still read the mem.