when you installed eCart, data Bridge and design Exteder, was eCart the last one to be installed? It should _not_ be installed last, you should make sure to install either design extender or data bridge after installing eCart. I would recomend uninstalling eCart and data bridge, then Delete the Dreamweaver Cache file from the users configuration folder:
and reinstall eCart first, then Data Bridge.
in some cases, i have seen the checkout wizard stall on creating the checkout pages till you click into the eCart object widow,try clicking in the eCart object widow if you continue to have the problem.