So... the one that was in the case already didn't fit the motherboard so you took it out and now you can't get the one the came with the motherboard in?
It can be tricky the first time if you're afraid of breaking something (I know it can be).
If you haven't done it before, just kind of smash it in

. Put the narrow side in and line it up with the hole in the case on the end it should go to. Then get the long sides to go in as well as you can. At this point, it should be tough to get the other end in... just push really hard or lightly smash it in with something

. Be careful not to cut yourself on it though.
If that doesn't work, let us know.