The problem, I guess, is the space, necessary for argument separation. Following settings do not work:

mycomOnMButtonClick "C:\2xExplorer.exe" "\ddrives\d"
mycomOnMButtonClick "C:\\2xExplorer C:\\"
mycomOnMButtonClick "C:\2xExplorer C:\"
mycomOnMButtonClick "C:\2xExplorer" "C:\"
mycomOnMButtonClick "C:\\2xExplorer" "C:\\"

while this works:

mycomOnMButtonClick "C:\\2xExplorer.exe"