Is this done by the rotating knob on the dispenser?
Yes, just a quick blast for a second or so between pours, or after a few if you're having more than one in an evening, you should be able to hear the gas going in until it makes a sort of honking noise.
The knob should be at minimum when you put in a new bulb. When you screw in the holder, you should hear & feel the gas escaping briefly as the bulb is pierced before you've quickly tightened it up all the way.
At that point you shouldn't hear any gas escaping unless you turn the knob, if you do then you may have a leak. Check the holder is fully tightened, check the dip tube is snugly tightened (not too much in case you break it), maybe smear a thin coating of vaseline around where the dip tube goes through the keg bung.
I've occasionally had issues with not being able to tighten the bulb holder enough to stop gas leaking from the bulb itself, I think certain brands are a slightly smaller size, so I stuck a bit of cardboard or padding in the bottom of the bulb holder to give a bit more purchase.
Did you leave a bit of headspace in the keg when filling? If it's too full there will be nowhere for the gas to go.
The CO2 part of the tap should work the same without being inserted into a keg, as long as the knob is at minimum it should be sealed, and if you turn the knob you should hear the gas. So if all else fails, you can try pulling the tap out of the keg and trying that.
All that said, there should have been plenty of pressure from the sugar to begin with, sometimes too much, so if it's totally flat that sounds like a leak, or it never carbed up in the first place, or if it's a red & black bung maybe it was left in the open position.
