The 5 levels are:
+7 dBm, (5 mW)
+15 dBm, (32 mW)
+18 dBm, (63 mW)
+21 dBm, (125 mW)
+24 dBm, (250 mW)

The buttons select the lowest, middle, and highest:
+7 dBm, (5 mW)

  +18 dBm, (63 mW)

  +24 dBm, (250 mW)

For many applications, the middle level (63mW) is a good choice.  This is comparable to the highest power level available from 2.4GHz RC4Magic, and still provides better range because lower frequencies propogate better and travel further than higher frequencies.  (900MHz is much lower than 2.4GHz; 2.4GHz = 2400MHz.)

Selecting power level on all DMXio devices is done using Func/ID0.  That is, the Func button acts like a Shift button while you tap ID0.  The RF Power indicator, the red LED, blinks faster for higher power levels.  Slowest blink is lowest power, of course.