how to add button to Finder toolbar to call python script with current folder as argument?
See if this works.

Download the Get first Finder Window Path BETA 10.9 automator action from from my blog : my_very_first_automator_action 

The action returns the path of the front most finder window. It does not take any input from above actions. The post explains it all :-)

Create a new Application document in Automator and add the Get first Finder Window Path to it. Then just add you Run shell script after it.

Save as Application. And then do the drag and drop.

I would test it with your code but I do not have the python slideshow...



Note from Apple documentation in regard to Third party Actions.


  Installing Actions When your action has been thoroughly debugged and
  tested, build a deployment version of the bundle (using the proper
  optimizations). Then create an installation package for the action (or
  add the action to your application’s installation package). The
  installer should copy the action to /Library/Automator or
  ~/Library/Automator, depending on whether access to the action should
  be system wide or restricted to the installing user.
  
  Instead of installing your action separately, you can put it inside
  the bundle of your application, especially if the action uses the
  features of that application. When Automator searches for actions to
  display, it looks inside registered applications as well in the
  standard Automator directories. The advantage of packaging your
  actions inside an application is that you don’t need to create a
  separate installation package to install the actions. To install the
  actions, users need only drag the application to a standard location.
  
  Action bundles should be stored inside the application wrapper at
  Contents/Library/Automator. Thus, if your action is MyAction.action
  and your application is MyApp.app, the path inside the application
  would be:

MyApp.app/Contents/Library/Automator/MyAction.action

  
  You can either manually copy an action into this location (after
  creating the necessary subdirectories) or you have Xcode copy it using
  a Copy Files build phase. If you copy an action into an application
  bundle but the application is already installed on a system, you must
  get Launch Services to recognize that the application has new content
  to register (that is, the new action) by changing the application’s
  modification date. You can do this by entering the touch command in
  the Terminal application.

sudo touch /Applications/MyApp.app

  
  Or you can rename the application in Finder to something else, change
  it back to the original name, and then launch the application once.