Yup, you'll need a level converter (you can build a simple 5V -> 3.3V using a resistor divider and 3.3V -> 5V using a transistor as well).
As for 5V power - if you use USB to power the scout, you can use the VUSB pin to draw 5V from. Otherwise, you'll have to use an external 5V supply. In the latter case, you can also connect that supply to VUSB so the scout can charge / run from that supply.
Hope this helps!