You already can. I just downloaded it for my 8.9 tab the other day. https://market.android.com/details?id=com.slingmedia.slingPlayerTablet&feature=search_result#?t=W251bGwsMSwyLDEsImNvbS5zbGluZ21lZGlhLnNsaW5nUGxheWVyVGFibGV0Il0.
If you are not able to download the SlingPlayer for Android Tablets, make sure that your device meets the requirements. If the problem is that you are not able to connect to your Slingbox, verify that your Slingbox is supported.
The following article will show you the requirements for this application: