The Actions are grouped together in groups of similar functionality.  You cannot add or remove Actions from the tree and the only thing that is allowed is expanding and collapsing the nodes of the tree as well as using the Enter or Insert keys on the keyboard to create a new Action Item.  Double clicking with the mouse will also create a new Action Item.  The new Action Item will be created above the currently selected item, i.e. it will push the selected item down and be created on the currently selected line.

You can change the font setting of the Action Tree from the Build Automator Options window.  There is currently no popup menu for the Action tree.