Okay, so I checked my 2003 700 twin and my 2003 400 and they both work the same way.
Set the parking brake
Put transmission in neutral
If you had to turn the key on to see what gear you were in, turn it back off
Wait 10 seconds
Hold down the yellow reverse/override button
Turn the key on, it the display should be in diagnostic mode
Use the button to cycle thru the options, you should see battery voltage, oil pressure light, and the third option should be tachometer to set idle speed
To get out of diagnostic mode, turn off the key, or put it in gear
You cannot view tach while the machine is in normal operation mode, only diag to set the idle speed...