Error while registering xulrunner: cannot open shared object file libgtk-xll-2.0.so.0

I'm installing on Linux 12.04.

After unzipping, I'm running

./xulrunner --register-user from within the xulrunner folder, and I get this message:

./xulrunner-bin: error while loading shared libraries: libgtk-x11-2.0.so.0: cannot open shared object file: No such file or directory

I'm not sure how to continue. Any help would be welcome, thanks!