1. Add the UIEventsToPlayMakerFSM script to the NGUI Button.
2. Drag the PlayMakerFSM that you want to send events to into the Target FSM field.
3. Add the NGUI events you want to respond to in the FSM (E.g., OnClick etc.)
4. Respond to the events
I've attached a screenshot that shows the final setup.
You only need to use Get Event Info if you need more information about the event.
Some docs on writing actions: https://hutonggames.fogbugz.com/default.asp?W166
You can also find a lot of tips on making custom actions on the forum, and Playmaker ships with the source code for hundreds of actions as examples.
I haven't spent a lot of time with uScript... In theory Playmaker and uScript should work quite well together