Simply download and install the MacOS drag-and-drop installer for your system from http://www.OpenMS.de .
To use TOPP as regular apps in your shell just add the following lines to your ~/.profile file. An adapt the first line to point to the folder where you installed OpenMS (e.g., /Applications/OpenMS-1.10.0)
| OpenMS / TOPP release 1.11.1 | Documentation generated on Wed Jan 29 2014 23:31:57 using doxygen 1.8.6 |