Look on the back side of the P/S pump for a 14mm bolt... Loosen this and the pump will move freely. There may also be a 10mm bolt on the upper side of the rear, this will move the pump side to side to tension the belt if so equiped.
Have the tranny serviced ASAP, otherwise you'll have to do it anyway as well as pay for the tow.
just my $0.02,
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles