Chosing the right Hall effect sensor

You're probably suffering from bounce. If you were to connect an oscilloscope to the Vout pin (I suggest you try this) you might well see something like this:

When you plug or unplug the jumper, it will be almost impossible to do this without causing several unintended pulses. I recommend not trying to program it by using jumper wires.

By far the best way to do it is to use a microcontroller and some transistors. However, if you really want to do it by hand, using this circuit might help:

Here, you can press S1 to pulse Vout to 27v, and S2 to pulse it to 3v. C1 will help to remove any switch bounce. I don't guarantee that this will work though.

Using VpH as an example the current will be limited to around 90mA by the resistor R7. The datasheet says the minimum current guaranteed to blow a fuse is 300mA and even recommends a 0.1uF capacitor to ensure enough current is available.

A simple solution more likely to work would be to scrap the resistor divider and use something like an LM317 adjustable regulator and switch the voltage set resistor shown as R2 in the typical application circuit instead. Using 1% resistors should keep you well within the tolerances given in the datasheet.

Also take note of markrages answer, switch bounce is likely to be a problem so ideally apart from a more robust supply you should be switching voltages from a microcontroller. The LM317 datasheet shows Iadj is limited to less than 100uA so you could achieve that using any form of transistor or analog switch that doesn't introduce too much resistance.