How do I use this voltage regulator with a digital pinout?
It's probably too late for OP, but I'll give the answer to the actual problem: How can a 317 regulator be switched to a low voltage under control of a digital signal. The answer is actually shown in the linked datasheet:



When the "TTL control" signal is high, the lower resistor in the adjustment divider is effectively shorted, and the output voltage will drop to about 1.2 V (maybe 1.4 V, accounting for the Vce-sat of the transistor). 

For a 3.3 V logic signal, the 1k resistor in the base of the transistor may have to be reduced. 

If the "reduced" output level needs to be higher than 1.2 V, an extra resistor could be added at the collector of the transistor.