Plugin Registration Tool 2015 can be downloaded from CRM 2015 SDK.
Workflow Registration Steps
Run the Plugin Registration Tool from SDK(Path:SDK/Tools/Plugin Registration/Plugin Registration Tool)
Create New Connection
Register your Organization by providing required information and credentials.
Click on Register Button and select Register New Assembly.
Click on Browse button to select .dll file and click on Load Assembly.
Select the Select All Button to check workflows to register and then click on Register Selected Plugins.
Click on OK.Now the dll file with workflows got registered.
Registration process is now completed.