Installing Virto SharePoint Workflow Activities Kit for SharePoint 2007
You should activate the feature for desired web applications, before using in SharePoint Designer, otherwise SharePoint Designer would block adding activities to the workflow. Run “SharePoint 3.0 Central Administration“.
Go to “Application Management” –> “Manage Web Application Features”
And activate the feature for desired web applications (usually it's Sharepoint - 80 or Sharepoint - 443).
If you have a non-English installation of SharePoint, you may have to copy to copy VirtoWorkflow.ACTIONS from [SP HiVE]\TEMPLATE\1033\Workflow to [SP HiVE]\TEMPLATE\"YOUR LCID"\Workflow.