I'll assume you already know how to eval code (Cmd/Ctrl-enter), how to open the command bar (Ctrl-space) and how to open files with the navigator (Cmd/Ctrl-o). If you don't, start with the Light Table tutorial.
Add LightTable/src to your Light Table workspace and open src/lt/objs/jump_stack.cljs. Hit eval (Cmd/Ctrl-enter) somewhere in the file to start a ClojureScript compiler. When it's finished starting up it will ask you where to send the emitted JavaScript code - choose Light Table UI from the menu. Now you should be able to eval ClojureScript code inside your current Light Table instance. Try something simple like (js/alert "foo") to make sure it works. Generally, we eval code as we write it and only compile with lein cljsbuild once if we need to restart Light Table.
The new Light Table release supports auto-complete (Tab), inline docs (Ctrl-d) and jump-to-definition (Ctrl-. to jump and Ctrl-, to jump back) for ClojureScript and Clojure vars, all of which are very useful for exploring the codebase. In ClojureScript these features are only aware of vars that have been eval'd in the current compiler process, so be sure to eval the ns form at the top of the file to get the full effect.
For hunting down behaviors, objects and other things that don't live in vars use the searcher (Cmd/Ctrl-Shift-f). If it isn't clear how to use a given function then using the searcher to find examples will also help.
Finally, use the documentation searcher (Ctrl-Shift-d) for full-text search over the names and docstrings of all known vars. Most of Light Table doesn't have docstrings, but this is still useful for library code.