Hmm... well I tried adding the button code to and EZ page and then adding that as a sidebox. It didn't work as I expected and instead the box appeared as a list titled "Important Links" and then the name of my EZ Page under that. If I clicked on that, then the browser attempted to load the button but couldn't. I'm guessing it's because the mibew directory is not a sub directory of the zencart directory.
I'll have to see if I need to move the mibew directory inside the store directory or whether I can point the html to the root to look for the mibew stuff.
I'm sure I've done this with ZenCart before and just don't remember how, but can you have a sidebox which just has same html in it rather than a list of pages?