When creating custom avtivities/actions to be used in SharePoint designer you may get an error stating that the workflow may be saved, but cannot be run. If this is the case it may be that there is "garbage" in either your web.config file, or there are *.actions files in your Workflow folder that references non-existing librarys. I got the error when retracting an old solutions containing custom actions that DID NOT clean up the file structure.
If you custom actions show up in the lists of available actions - and that is the onlyt thing that is displayed - check that uou have activated your feature (if deployed through one).
AND you have to make sure that every library/dll referenced in your actions/activities actually are installed on the server. Even if your code compile well it may not run ....
Of course the hardest one to find: TYPOS!