I get errors when trying to install GTK+3 on my Mac OSX 10.9 system
Forgive me if this is a silly question but do you have Xcode installed?

The OSX SDK should be installed on your system within the Xcode application bundle. You should be able to find it at /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/.

If Xcode isn't installed you can install it for free from here.