Most Unix / Linux operating system distributions, as well as Mac OS X, include Tcl/Tk. If not already installed, you can use your system's package manager to install the appropriate packages.
If you don’t already have Tcl/Tk Aqua installed on your computer (most MacOS X installs have it), you will get this error message. To resolve it, simply go to the website for Tcl / Tk Aqua and download the latest version:
http://www.categorifiedcoder.info/tcltk/
If you have an older version of Tcl / Tk Aqua, you’ll benefit from upgrading.
More information:
http://code.google.com/p/git-osx-installer/issues/detail?id=41