Look to see if the 30 amp panel is fed with a two wire or a
three wire. If it is a two wire then you are out of luck. If the
panel is fed with a three wire then the panel should have the
neutral terminated in the panel. It is this neutral that you need
for 120 volt connections. You didn't state how many panel slots
there are in the panel. If you are able install a 15 amp breaker
into the panel and connect the wires going to the load. The black
wire will go to the breaker and the white wire will go to the
neutral bar in the panel.