Depends on the dry kit. There are a couple different methods. Some like the Cold Fusion and the Nitrous Works setup have the nozzle in front of the MAF, so that when the cold nitrous hits the MAF it tricks the computer into thinking it's getting a whole lot more air then it is, and adds more fuel. This setup is garbage in my opinion. It's extremely inaccurate and risky. Then there are ones that interrupt the signal from the MAF, and that tells the computer to add more fuel. Then there are ones like Noobz was talking about where it has a chip plugged into the computer that adds the fuel. I don't mess with any of these setups because I don't think any of these methods are terribly reliable. In my opinion it's all of these setups that have given dry setups a bad name.
The only dry setup I like is the 05115 setup from NOS. It's patented technology which is why no one else uses it. Other companies either make a different style of dry kit, or don't make one at all, and try to convince you that wet is better. The 05115 adds the fuel by spiking the fuel pressure in the fuel rail. The injectors pulse at the same rate as when the engine is running N/A, so it doesn't effect your N/A tune at all. They just deliver a lot more fuel because the pressure is higher.
When the wide open throttle switch is activated, it opens a primary nitrous solenoid. The nitrous then flows to both a fuel pressure booster (NOS calls it a fuel solenoid but it really isn't), and a second nitrous solenoid. The fuel pressure booster reduces the nitrous pressure, and then channels it through a line that T-s into the line between the manifold and the fuel pressure regulator. In the T, there is a jet, so that you can moderate the rate at which the nitrous goes in. A small amount will always bleed back to the intake, but the jet will keep a constant pressure on the top of the fuel pressure regulator. The pressure forces down on the diaphram in the fuel pressure regulator which causes the fuel pressure to spike to an exact pressure. About 65 psi for a 75 shot, and about 95psi for a 150 shot. You adjust that pressure with a jet just like you would on a wet kit. There is a fuel pressure switch on the fuel rail that closes the circuit to the second nitrous relay when the fuel pressure spikes. I really like this setup, because you can't get the nitrous unless you already have the fuel. It sounds a lot more complicated than it is. It's actually pretty easy to install, and very reliable.
Kurt