Binary releases and source code for oMGUI can be downloaded from the Sourceforge.net repository.
There are currently no binaries of oMGUI1 available. Please refer to the build instructions for your platform (WindowsBuildInstructions, MacBuildInstructions, LinuxBuildInstructions) to compile the code from source.
oMGUI2 is currently 'vapourware' i.e. it does not exist other than in the vision of the developers. To see screenshot mockups of oMGUI2 go to the ScreenShots page.