The SimpleCmds are stored as Python script files under the simplecmds directory. This directory is dynamically scanned on startup of the QubeGUI whereupon all Python scripts within that directory are then loaded. The simplecmds directory can be found with the executing QubeGUI scripts directory.
If running the compiled executable version of the Qube GUI (WranglerView), then it is located in, depending upon the platform:
Windows: C:\Program Files\pfx\qube\bin\simplecmds
Linux: /usr/local/pfx/qube/bin/simplecmds
OS X: /Applications/pfx/qube/qube.app/Contents/Resources/simplecmds
any of which can be found by going to File > "Open SimpleCmds Directory" from the GUI, itself.
If you are running the qube.py script, you will find the simpleCmd directory in:
$QBDIR/api/Python/qb/GUI/
where $QBDIR is the location of the Qube directory. This can also be found by going to File > "Open SimpleCmds Directory" from the GUI, itself.