1. Make sure that following record exists in System.Workflow.ComponentModel.WorkflowCompiler\authorizedTypes section of the web.config file.
<authorizedType Assembly="DP.Sharepoint.Workflow, Version=22.214.171.124, Culture=neutral, PublicKeyToken=0298457208daed83" Namespace="DP.Sharepoint.Workflow" TypeName="*" Authorized="True" />
web.config file can be found in c:\inetpub\wwwroot\wss\VirtualDirectories\*Your_Directory*. Please select the directory name carefuly 'cause it's one of most frequent reasons why activity doesn't work.
2. For non-english installation of sharepoint you need to copy DPWorkflow.ACTIONS file from C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\1033\Workflow to C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\*Your_Country_Code*\Workflow
. For example for Germany *Your_Country_Code* will be 1031.
3.Check that feature is active in Sharepoint Central Administration -> Application Management -> Manage Web Application Features -> "Additional Sharepoint Designer activities". Please pay attention on what web application feature is activated
'cause it's also a common error. Web application name can be found in left upper corner.
4. Close Sharepoint Designer, restart your web site in IIS throug Start->Administrative Tools->IIS Manager->WebSites->*Your_Current_Sharepoint_Site*.
Hope it will help you.
With best regards,