You have to drag the Text Mesh component from the Unity inspector onto the Target Object field. You'll then see all the public properties in that component.
Object in this context refers to a UnityEngine.Object, not a GameObject. This means you can manipulate anything derived from UnityEngine.Object. Create an Object variable, select it, then look at the Object Type popup to see the full list!
Does that help? I need to make a quick screencast to show this off. It adds a lot of power to Playmaker, but I think most people will try to reference a GameObject instead of Components etc.