There are two different ways to extend krpano in the browser. You can write javascript that "lives" in the html page, and communicates with the krpano viewer, or you can write a plugin that lives inside krpano (don't confuse plugin with the <plugin> node; plugins can be used as <hotspot> too).
A plugin needs to be both flash (as3) and js if you want to support both platforms. There is no automated translation between the two, but depending on what you want to achieve, the code will be very similar.
I don't use an IDE myself, but I can recommend using a (git) repository to keep track of your progress while making plugins.
Finally, if you want to learn from examples, you could look at my collection here:
http://fieldofview.github.com/krpano_fovplugins (descriptions)
https://github.com/fieldofview/krpano_fovplugins (code)