GTK+-2.0 and X11

So, I downloaded, compiled, and installed a program and it runs fine. However, it also comes with a GTK+2 frontend and I wanted to see if I could get that running, just for the heck of it. When I run ./configure, it rumbles along for a while and then tells me it couldn't find pkg-config, and it gave me a website to go and get it. So I did, compiled, and installed it. Now it halts on this:

checking for pkg-config... /usr/local/bin/pkg-config
checking for gtk+-2.0 >= 2.0.0... Package gtk+-2.0 was not found in the pkg-config search path. Perhaps you should add the directory containing `gtk+-2.0.pc' to the PKGCONFIGPATH environment variable No package 'gtk+-2.0' found
configure: error: Library requirements (gtk+-2.0 >= 2.0.0) not met; consider adjusting the PKGCONFIGPATH environment variable if your libraries are in a nonstandard prefix so pkg-config can find them.

I tried installing pkg-config under X11 as well, and trying to configure this program under X11, but it gives me this:

checking for pkg-config... no
* The pkg-config script could not be found. Make sure it is
* in your path, or set the PKG_CONFIG environment variable
* to the full path to pkg-config.
* Or see http://www.freedesktop.org/software/pkgconfig to get pkg-config.
configure: error: Library requirements (gtk+-2.0 >= 2.0.0) not met; consider adjusting the PKG CONFIGPATH environment variable if your libraries are in a nonstandard prefix so pkg-config can find them.

This is basically the same thing I got in Terminal.app except it doesn't see pkg-config. Do I need GTK+-2.0, or do I just need to mess with some path files, or both? If I need it, can I just go to the GTK website and download it, or do I need a special OS X version? Also, should I install it under Terminal or xterm, or does it matter? And why doesn't xterm see my pkg-config installation?

Thanks. =)