First and foremost, you need a IR receiver that is compatible with MS Media Center (for either Windows or Linux).
eg. This will work:
http://www.takealot.com/computers/c...ol-for-mce-media-center-edition-black,7273135
Then when you configure the Harmony 300 you need to assign a button to a Windows Media Center receiver, and tada it will work with XBMC. Thats how I do it on Linux, I didn't install any funny software nothing, it just worked out of the box.
ps. I don't know how well that specific receiver work, I have a receiver built-in on my HTPC case which is a MCE compatible receiver, using my Harmony 300 with it, after my Harmnoy 700 was stolen (still miss it). However in my experience, the Harmony remotes work much better than the original I got thinking about signals strength and angles at which it work.