Thank you for posting to the forums! I understand that you are trying to mirror the image from your Slingplayer app to the Roku.
Although the Slingbox PRO is not supported to work with the Slingplayer app some people do get lucky. This being said, the streaming to the Roku is not supported with the Slingbox PRO.
Here is a popular article about the Roku streaming:
The Sling Moderation Team