Im not an expert with playmaker or NGUI but I found it pretty easy do do those transitions you speak of. What you have to do for a button to work is make sure you add the event forwarder script in the playmaker/ngui package you got. Once thats attached to the button, you create a playmaker fsm with a global event. Its the global ngui button clicked ( im at work now so I dont rememebr the name, but its in the global/custom/ngui section, should be easy to spot.) Now you add whatever event you want to fire, a pause, level load, what ever you want to fire when you click that button.
The main things to make sure is that NGUI button has its collider, is on a clickable layer, and you have the eventforwarder script on it along with the ngui/playmaker delagates prefab in your scene.
