Mac OS X

Homebrew

To install GtkExtra-3 from Homebrew, use:

brew install gtkextra

 

MacPorts

To install GtkExtra-3 from MacPorts, use:

sudo port install gtkextra3

 

Note that there is also a package named 'gtk2-extra' in MacPorts, which refers to GtkExtra-2, 
which is no longer maintained by the developers.