Keep in mind that the <iframe> is interpreted by the browser, not the server. So when the browser goes to
http://localhost:8100/ and you tell it to go up two levels, it has nowhere to go (nor would you want a browser to be able to arbitrarily go poking around your hard drive!).
There are a few different ways I could see tackling this. If the .htm file isn't going to change, the easiest would be to simply copy it into a subdirectory of your app called "www", and do tags$iframe(src="page.htm", ...) [note that no "www" is needed in the src]. If you do need to keep that page where it is, and don't mind that entire directory being accessible over the web, then take a look at ?addResourcePath.