I dont know scripting at all myself, but i think I know how it can be done. If you could set up a float (or int i think) variable to be the score in your script.
Then with the 'game over' fsm open, drag and drop the script component from the game object onto the state's action list. When you drop the script there select 'get property', in the small menu that appears.
In the 'get property' action's drop down menu, i think you should be able to choose the 'score' variable from the script (though ive never used get property on a script myself). Save the score variable from the script into a playmaker variable, every frame if thats an option.
Then add a float/int compare to compare the variable you just saved from the script...
When the score is 0 or less send to the game over state (or whatever numbers you have in mind).
Also you can reverse this using 'set property'. So to make a bool for the script, setup a bool in playmaker ( for example 'game over= true' once the score has reached 0) Drag and drop the script component from the game object, into the actions list on the game over state. Choose 'set property'. Then setup the action so the playmaker bool value sets the bool value in the script.
Hope my ramble helps lol. Let us know how you get on.