I looked long and hard (reallly long and reallly hard) for an x86_64 version of the SDL.dll file (Simple DirectMedia Layer) but found it nowhere. I ended up compiling it myself with mingw64 and figured maybe some other people might benefit from my hard work and dedication.
Get it here, and place the DLL in %WINDOWS%\System32\
Keep in mind this was compiled on my system, so it may not work on yours, although I’ve tested it on three different x86_64 Windows 7 machines and it works fine, just fine. Lispbuilder-sdl loads the DLL and I’m able to do OpenGL programming in Common Lisp with it (at least in ClozureCL).