To set your input device as StageSound to use with StreamYard ensure StageSound is opened and is turned on. Then start streaming on StreamYard, in the initial set-up menu select the Cam/Mic cog wheel.
Open the Audio settings using the menu on the left, then select StageSound from the Mic dropdown menu.
For best possible sound, also ensure the "Disable audio Processing" box is ticked.
When you have finished streaming and no longer want to use StageSound on StreamYard then follow the above steps again to access this menu, this time selecting your usual microphone.