Provided it's an object in the scene (i.e. an instance of the prefab), it should be visible above the actions. Hard to miss. Otherwise, it might be a setting (look into preferences, bottom in the PlayMaker editor window). If it won't show up, you can just edit prefabs by selecting the prefab and in the inspector on the right, where you should also see a button to edit the prefab.
If you select a prefab in the project, and not in the scene, you automatically edit the prefab.
