Double check the installation (if you still have the manual). I have a Stebel Magnum compact, too (that I installed myself) and it's oriented 180° on the vertical plane to yours, so that the two leads are facing down.
This may or may not affect your horn's perfomance (certainly it has to be within about 15° of the vertical and if it's not the compressor won't work well). I'm not sure about it being totally upside down.
Secondly, make sure that they installed the horn with the relay that was provided. Some people choose to forgo it (as it takes extra wiring and some space), but this can lead to a dead horn -- sometimes within just a couple of months -- as it gets too much juice.
Here's a pic of my installation (in the cowl glovebox, with the 18 amp/hour sealed lead acid battery underneath it).
There's extra wiring there for the twin Hella lights I have installed on the front rack:
The Stebel air horn is definitely MUCH louder and effective than the strangled duck stock horn. It can take a quick second to spool up (the compressor) but it's definitely loud! Stebel also makes a non-compressor version that's also very loud and probably has a quicker response time.
To activate it I just set up a cheap doorbell by my left foot. I didn't want it anywhere else, or to mess with the original wiring of the scooter (a non battery 6V setup). I had tried other switches, but they were hit and miss. The doorbell can be easily replaced for under $5 if it stops working.