Windows Registry Editor Version 5.00
The above saved as a .REG file and imported into the registry will run your app and pass the full path of the selected folder to it as the first argument. Here's the content of MyApp.bat for testing:
for %%a in (%*) do echo Argument = %%a
Remove the MultiSelectModel line if you want the context menu entry to show up even when you select multiple folders and right-click.
However, this is more of a manual/hack-y method. I highly recommend you read the Creating Shortcut Menu Handlers MSDN article and ask over on StackOverflow about how to make your app's installer properly register your app as a valid context menu handler.