You could not add any activity from Virto Software Workflow Activities Kit because, they are not registered as authorized types in your web.config.
Virto Software Workflow Activities Kit adds web.config modifications on feature activation.

Solution #1:
1. Go to Central Administration -> Application Management -> Manage Web Application Features 
2. Activate the Virto Software Workflow Activities Kit feature for desired web applications (usually it's Sharepoint - 80 or Sharepoint - 443).
3. Restart SharePoint Designer

Solution #2:
1. Open your web.config (c:\inetpub\wwwroot\wss\VirtualDirectories\[port]\web.config) 
2. Add xml line below 

For SharePoint 2007:
<authorizedType Assembly="Virto.Sharepoint.Activities, Version=1.0.0.0, Culture=neutral, PublicKeyToken=7b70ddd211dc63c8" Namespace="Virto.Sharepoint.Activities" TypeName="*" Authorized="True" />

For SharePoint 2010:
<authorizedType Assembly="Virto.Sharepoint.Activities, Version=14.0.0.0, Culture=neutral, PublicKeyToken=7b70ddd211dc63c8" Namespace="Virto.Sharepoint.Activities" TypeName="*" Authorized="True" />

For SharePoint 2013:
<authorizedType Assembly="Virto.Sharepoint.Activities, Version=15.0.0.0, Culture=neutral, PublicKeyToken=7b70ddd211dc63c8" Namespace="Virto.Sharepoint.Activities" TypeName="*" Authorized="True" />

to section
System.Workflow.ComponentModel.WorkflowCompiler\authorizedTypes

3. Restart SharePoint Designer