After testing the demo, I can confirm that Sundog is working perfectly with Reaper 5.01 x64 on win8. Here's how you can get it to work.
First, download LoopBe1 MIDI driver and install it:
When that's done open up Sundog and click on the 'settings' menu at the top, then 'preferences' and set 'MIDI out device' to LoopBe Internal MIDI and click on 'ok'. Now set up one of the instrument channels to play something whilst taking note of which MIDI channel it's set to (this is easily changed if necessary). You might also find it useful to set the 'instrument' parameter to off, as this can mess with the VSTi that you'll be using to play your musical part. That's about it for the Sundog side of things now on to Reaper.
Assuming you've done all the above, and you have Sundog open and ready to go, open up Reaper and go into the preferences menu and under MIDI devices> MIDI Inputs, right click on LoopBe Internal MIDI and set it to 'enable input'. Now back in the arrangement view, insert a new VST instrument, say a piano or something, and set the record arm on the track to on. Now click on the 'Input' drop down menu on the track, then 'Input: MIDI', followed by 'LoopBe Internal MIDI' and set it to the same MIDI channel as the instrument in Sundog. To test if everything is working, go back into Sundog and hit the play button, and you should hear your piano play the part from Sundog.
Hopefully that makes sense and will get you up and running, but if not, give me a shout if you get stuck.