In my setup I had 3v from the Pi hooked up to BAT+ on the Trinket, and GND to, well, GND. Not sure if this is right, since I thought BAT+ went through the 3v regulator, but it appears to work. I reckon 3v could go right into the Trinket's 3v pin though.
Not sure what else could be amiss. Did you have as much trouble compiling it as I did? ( undefined NEO_KHZ400 mostly )