I tried them now and was able to get up a Wat VM. But I haven't been able to figure out how to run macros in there.
I wish there were examples. Snippets you can copy paste that do lots of useful stuff like http://ycombinator.com/arc/tut.txt. It shouldn't take anyone more than 10secs to find how to do things. It's disheartening to hear such good work remains unused only because there are no examples but I suspect it's the truth. That's what happened to me.
Language designers think some cool esoteric feature is the killer feature of a language but a more useful feature might be examples: an implementation of an application developers want, like Arc did by providing a webserver and a blog.arc. If you are writing a language, ship a useful app with it, like C did with UNIX.
I would look at the boot.js file. It's kind of like arc.arc is for arc, in that it is a lisp-based bootstrap of all of the basic functions made available in the default environment.
It should give you basic examples of how to write code in wat, as well as show you which functions are defined. Of course, most of them don't actually have examples in the file, as it's merely defining the standard library, but it's a start. There's also test.wat, and the authors blog: http://axisofeval.blogspot.com/search/label/wat (which unfortunately mostly uses the underlying json representation for most of its code).
And the 'killer feature' of wat.js is not that it has first class environments or fexpr, which are admittedly very cool, but that the core language implementation that supports all of these and more is only a few hundred lines of code. There's more in the repository of course, but all of that is to provide the standard library or add sexpression parsing, etc. This makes wat a very good minimal platform for building a custom language.