I created for SharePoint Designer 2010 in Visual Studio 2010. My custom action appears in Designer in the action list just like it's supposed to, but when I click on it it does not appear in the workflow. I don't get any error messages throughout the development process, and Designer seems to have no trouble with knowing the action is there and there is an assembly associated with it.
Solution :
I have checked application web.config file and found that <authorizedType Assembly="DocToPDFActivity, Version=1.0.0.0, Culture=neutral, PublicKeyToken=2478de059b6051f4" Namespace="DocToPDFActivity" TypeName="*" Authorized="True" /> is configure in it.
So Just i have added it and reset IIS.
After I have checked Workflow action UI and it appears.
Cheers.
