I just followed the few basic steps suggested by Cristian, did a bit of basic trouble-shooting and voila – an updated version of the QuickWatchGadget is ready for CMS 6 RC1 (And hopefully also for the final release).
It’s an easy install – just download the file and install it as a module through deployment center – and you are ready to start debugging your site with real-time code compilation and execution in your very own quick watch window.