I was getting this error message in Visual Studio 2008 when adding a custom activity to an existing workflow.
Solution
This worked for me, your mileage, may and probably will vary.
It was necessary for me to un-gac the workflow project, then the custom activities could be added to the design surface as expected.
Edward Wilde 
Thank you! Saved me a lot of time!
Thank you so much! I’ve been banging my head about this for the whole day…
How can one develop for sharepoint then, since the workflow libraries go into GAC everytime you deploy?
Thanks a lot…
Thank you very much!! I couldn’t figure out why I was getting this error. That solved the problem.
just close the visual studio and open again… worked for me
you saved my life. Thanks a lot.
Thank you, sir!
Crazyyyyy simple and logical. Thanks for sharing this.
thank for this it saved my time