This is most likely due to the clock source. Use the SYNC button to cycle through the different clock sources (INT, MIDI, USB, CLK). If the setting is not set to internal (INT), you will not be able to play any patterns until the RD-8 receives a clock from your selected source. This will be shown by the fact that the play button will flash. Once clocked externally, you will see the BPM counter adjust to the incoming signal. To play your patterns at the BPM selected by your counter, ensure the sync source is set to INT.
Keywords: Rhythm Designer, RD-8