Thanks rq3. I have used this switch with no problems, going from almost 5v to 0 volts. I was beginning to wonder if this device was a sensor and not a switch. I might have to rewire the device to an analog pin and use it's voltage. Just seen erco's reply, thanks. Will try another device when received (on order) and see what happens.
EDIT:
I forgot to say, I have read that the device is normally off until a magnet is next to it. Mine is high all the time until going low with magnet. ??
Yes, it is normally off, meaning that the output mosfet is in a non-conducting state, so that the mosfet drain (the "output" pin) is pulled up to the supply voltage by the resistor you choose. When the device is ON (a magnet near it), the mosfet conducts, and "shorts" the output pin to ground to the best of its ability. The higher the resistance you choose, the closer to ground it will get, because the mosfet doesn't have to shunt as much current to ground.
Open drain and open collector outputs are very common. You just need to reverse the logic in your own head. You are probably used to thinking that "off" is "low", and "on" is "high". This is just the reverse, that's all.
To interface the output to a Picaxe, you would use, say, a 100Kohm resistor between the output pin and the +5 volt supply. The output pin would then go to any Picaxe digital input (NOT an ADC input).
Your Picaxe code would then ask that pin what it was seeing. If there is no magnet, the Picaxe would see a "high". If there is a magnet, the Picaxe would see a "low". The output voltage does not vary with the magnetic field strength, it only switches from off (high) to on (low) when a sufficiently strong magnet is near.
If that's not what you are after, there ARE hall effect sensors that DO output a variable voltage with varying magnetic field intensity.