Gimp and Inkscape are free open-source pieces of software I use during the design process of my amps.
They both need X11 window management system, but Apple does not support X11 under OS X 10.8 Mountain Lion any longer.
You need to download and install XQuartz, following this link : http://xquartz.macosforge.org/landing/
After reboot, when you run Inkscape for the first time, it needs to know where X11 is, so browse folders and find it in the Applications->Utilities folder :
And … yesssss !
And Gimp works as well !