That sounds like you're getting a few things confused.
1. MIDI input and Control Surfaces have nothing to do with the ability to use your BR-800 as an audio interface.
2. You can either chose to use the BR-800 (if it has MIDI functions) as a MIDI controller via Preferences->Audio->MIDI Devices
or as a control surface (if it supports a control surface protocol REAPER understands), trying both will result in an error message (at least if both functions are using the same MIDI port).
3. Setting up the
audio device in REAPER is described in section 1.12 (page 23) of the User Guide. Please get it here if you don't have it yet:
http://www.cockos.com/reaper/userguide.php