Every thing went well and we were able to see the custom action in Actions tab in SharePoint 2010 Designer but when trying to add them, the action is not getting added.
This is the most common issue that everyone encounters. Here are the possible mistakes that leads to this problem.
1. Check if you have added the
<authorizedType Assembly="Namespace.ClassName, Version=126.96.36.199, Culture=neutral, PublicKeyToken="Your Public Key Token" Namespace="Namespace" TypeName="*" Authorized="True" />2. Check if there is not more than one character space between elements in .ACTIONS file
Remember that SPD caches the .ACTIONS file. Hence, after making appropriate changes, do an IIS Reset and Restart your SharePoint Designer.
If everything is correct, you will see that your custom action will be added to designer.
Happy SharePointing :)