If you need to force carb it, here's how:
1) once it's refrigerated, set the regulator to 30 psi.
2) while leaving the co2 line connected, pick up the keg. Hold it horizontally with one hand on top and one on bottom.
3) Tilt the keg by lifting the bottom up until you hear gas bubble up through it. Do that 20 times, then let the keg settle for 5-10 minutes with 30 psi pressure.
4) taste a sample, repeat 2) and 3) if necessary. It usually takes three repetitions to carb from still beer to moderately carbed. since yours is partially carbed, I'd take a sample after the first one. You can cut the number of tilts if you only need it to come up a little bit.
5) once you are satisfied with the carb, set your regulator to the desired psi for your temperature and carb level (10-15 psi at 36-45 F). it will take a day or two to equilibrate, but should be drinkable after a 10-30 minute rest.