The Plug-in registration tool is used register the custom code into the Dynamics CRM 2011 system.
What is a Plug-in: A custom code written to implement business logic targeted at modifying the behavior of the system. They are event handlers and execute in response to particular events being fired by the platform. A Custom workflow activity written is also a plug-in that needs to be registered in CRM system.
Registering a custom workflow activity with Microsoft Dynamics CRM Online is not supported by any means.
The Plug-in tool can be found here the executable is part of the CRM 2011 SDK and is called pluginregistration.exe
The First step is to create a connection, Click on the Create New Connection enter the Discover URL information. The end points for services available with CRM 2011 can be viewed by going to Customizations > Developer Resources.
Once connection is established the list of organizations are displayed:
Information Dynamics 