Hey,
I've read that it's possible find inactive objects with C#, so there might be an action somewhere for that (not in Playmaker by default). However, it shouldn't be too difficult to work around this issue. I'll list a few suggestions:
1. Attach the inactive object to a parent object and put "Don't Destroy On Load" on the parent instead. Then you can use "Find Game Object" to find the parent object (since it's active), and use "Get Child" to get whichever inactive object you want out of its children.
2. Create a new object with "Don't Destroy On Load" in the scene with the inactive object you want to find, then create a Game Object variable on the new object, and put the inactive object into the variable. Then you can find the new object using "Find Game Object," and then use "Get FSM Game Object" to get the inactive object.
These methods also have the advantage that you only need to use "Find Game Object" once, which can be a very performance demanding action in large scenes.
Hopefully that helps, good luck!