If you encounter this error while trying to write a game on a Windows 64 bit system using Slick2d, I can give you some insight into how to fix the issue. I’d ran into this issue some time ago, but I don’t remember if I posted about it or not. I solved the problem then by using a recompiled version of slick2d. It worked well, but I did something different this time around.
I downloaded the latest version of LWJGL and used it instead. The process of importing the libs is much the same except you use the new LWJGL and its natives instead of the ones included with slick2d. If you need help getting the imports correct, leave a comment and I’ll go into it in more detail.