There's two buttons labeled "Guide". One is after you tap "Remote" and then tap "All" (it will appear in the double row of all of the IR remote commands that can be sent, like this). This sends the "Guide" command to your TiVo via IR. The other button marked "Guide" that appears when you tap on an otherwise normal stream of video (shown here) gives you the Slingplayer-generated guide.
If the latter button doesn't show the Slingplayer generated guide, I'd suggest running through this Slingbox.com - I got a new set-top box, and I can't control my TV now to ensure your Slingbox is tied to the right cable/satellite provider.