First off, I want to say that I have it working (finally) on Firefox on Linux. So there should be some good news there - it's been tested on non-Windows :-)
Secondly, "invoking" is not entirely clear to me. So I want to expand on how it's working for me, which I presume is how it's working for everyone else.
Basically, you must right-click on an actual href anchor link, aka, <a href="http://(?:www)\.perlmonks\.(?:com|org)"> in HTML. The actual text above, "http://www.perlmonks.org/?node_id=12345", doesn't quite cut it, as I erroneously inferred. Thus, right-click here. Of course, if you're using a left-handed mouse, left-click there. Whatever. ;-) Then one of the options will be "make PM-link". Select that, obviously. Note that, at least here, it's not coming up with a keyboard shortcut (no letter is underlined).
What would be cool for extentions, holli, is if you did likewise for perldoc, doc, and cpan links. :-) Of course, isn't it always the case - start off with something simple and cool, and everyone wants to add stuff to make it ugly and bloated ;-} Regardless of whether you do this or not, I've got this version installed. Thank you!