In Arduino IDE –> Tools –> Board menu, select “Duemilanove or Diecimila”.
but note that is for the Rainbowduino 3.0, not the 2.0. Seeed has the incredibly annoying habit of deleting the documentation for their previous product versions. Still, it might not be a significant change so it's worth a try.
Beyond that, you might just contact Seeed Studio technical support.