Pretty sure Roku doesnt make or code that app.
Roku <> Apple - just because an app exists for the Apple platform doesnt mean its available for Roku or vice versa; its up to the app developer to develop apps for different platforms.
Did you verify that the app was available on Roku before buying the SS+?
Pretty sure it doesnt say Roku.
Contact Startup Show Tech if you want a Roku specific app.
However, it does mention AirPlay and mirror/cast support, all of which Roku supports on most models of the last several years, so use those processes.