First make sure you have a workflow document open.
If that simple mistake didn’t fix your issue, then close Visual studio and browse to the following folder:
“%USERPROFILE%\Local Settings\Application Data\Microsoft\VisualStudio\10.0”
Delete the files with extension “.TBD” in this folder. (These files are hidden and if you cant see them you will need to turn on the ability to view hidden files in Windows Explorer). Restart VisualStudio and your toolbox will be rebuilt after which you should have your WF toolbox items available (at least this did it for me).