A couple things come to mind.
1) did you activate the feature on the web app level in central admin? Just installing the solution adds the .xml files to the proper locations for it to show up in SPD, but the safe assembly entries don't get created in the web.config file until the feature is activated. I don't particularly like this behavior, but I'm not aware of a workaround. It becomes a real pain if an admin wants to make the activities available in one web application but not others.
2) did you activate/deactivate the feature multiple times? If so, you may need to manually clean up your web.config. I'm going to add thsi to the list of known issues right after I type this up here. Activating the feature will put the safe assembly entries into the web.config, but deactivating it will not remove them. I'm about 99% sure this is a bug in SharePoint itself. I seem to be able to remove entries from anywhere else in the web.config correctly, but this particular spot fails. I've seen third party workflow tools leave orphan entries in this same section. I've noticed that if I have multiple entries for the same assembly in the web.config, odd things happen.