Here’s a quick how to video.

Like it says on softimageblog.com:

p.s. : Cool people add fxtree operators by clicking them in the Operator Selector, and then middle-clicking several times in the fxtree to chose the 1) first input 2) second input 3) output (if necessary) and finnally middle click in the background to set the insert location. The Fx operator selector shows which connection is currently being selected. Select any optionnal inputs you want to pick by first clicking the checkbox next to it in the Operator Selector