The Actor Browser is available from the View menu or by clicking on the Actor Browser icon (a chess piece) from the toolbar along the top edge of the main T3Ed window.
This window allows you to find all of the actor classes available in the game, organised with respect to class inheritance. Therefore you have a standard tree-type structure on the left pane of the browser, and the properites of any given actor on the right. The slider at the top allows you to adjust the pane splitting.
Some actor classes have and asterisk before their name (eg. *Light, *PaganShamanCity) and this indicates that they are placeable in the game world.
Using the Actor Browser Edit
You find Actors by expanding the tree and browsing through. If you know the exact name of the Actor you want but can't find it you can type scc classvisiblename=ActorName in the command bar at the bottom of the main window (replacing 'ActorName' with the name you want, of course) and hit enter. The browser will expand the tree to that Actor.
Once you have selected a class, you can place it in the level by right-clicking on a surface, and the class you just selected should appear in the context menu as the option "Add (class) here", as long as it is a placeable class.