I think you just need to update your pack. That's what happened with me. Just quit the launcher, then reopen it, and after clicking "play" it should give you the option to update it. You can also probably update it from the pack settings
Wow. Thanks! I think it was a problem with me setting the "Add Waypoint" to my Right Shift button. I'm not going to try and to the LWJGL thing, out of fear of messing my game up. But Thanks for the Help! I really appreciate it.