de2 board expansion header

Interesting device, thanks for the reference. If you power it with 3.3V you can directly connect it to DE2, as Cris72 stated correctly.

But for stronger ultrasound beam/distance it needs to be powered with 5V. I think you can still connect it directly to DE2 with 5V power to the device, since 3.5V output from DE2 would be enough to switch input to '1', and DE2 has diodes and current limiting resistor on header I/O in order to be able to receive +5V. You may also use level shifting buffer, to convert +5V signal to +3.3V on signals going from sensor to DE2, but this is not mandatory.