${sourcepath} is blank too. I am using it as a Program parameter to a Quick Launch program.
That is because the QuickLaunch Bar is not a script.
It is like the QuickLaunch bar that you had/have in Windows on the taskbar, a place to quickly launch programs.
And you can drag and drop files on an icon and they will be sent as parameters to the program.
If you want it to interact with MC then you need to create a UserDefinedCommand and then add that to the Quicklaunch bar.
Here is a example, Create a new UserDefinedCommand of "CustomCommand" type
then enter
MC.Run CMD="C:\Bin\Notepad++\notepad++.exe" ARG="${sourcepath}"
Then Right click on QuickLaunch bar and select insert User defined command.
The Right click on the quicklaunch program icon for the command and select customize and then you can modify the icon.