I am having the same issue - the Slingplayer app remote works through the browser on watch.slingbox.com, and also works on the Windows Phone, iPad, and Android apps.
It just does not work on the Windows 8 app.
My setup -
Windows 8.1 Slingplayer app
Since support is not helping, and just tells us to post the question here, not sure what to do next.
For $14.99 it's a pretty expensive app for a remote that does not work.
No wonder the Windows 8 Slingplayer app only has 2.5 starts in the store
Come on Dish and Sling, get your act together and improve the Windows 8 app!
Update: here's something that worked for me -
1. Close the Windows 8 Slingplayer app.
2. Uninstall the app
3. Close the Windows Store
4. Re-install the app
Now when I run the app, the remote works.
The interface and features are still lacking compared to other Slingplayer apps.
One example: don't throw me to the guide when the app first starts - show my Slingboxes so I can just connect and watch my shows.
Instead the Slingboxes are burried under the settings > options > devices.
Please hire a designer who understands Windows 8 apps and UI in general and update this app.