The Slingplayer Mobile for Andriod should work fine with your Evo. However, it is possible that you're being blocked by Google's Andriod Marketplace filters. The Slingplayer Mobile is marked as a paid, copy protected application. So, you could be filtered out due to location, using an unrecognized operating system, or having a rooted phone.
You might want to check Google's ANdriod Market Support (http://market.android.com/support/) for further assitance, as there are workarounds available depending on your particular issue.