Let me see if I can help you out Pyrate;
1--First open up GarageBand.
2--After it opens up, you want to open a new track, do this by going to the track menu at the top of the screen and select new track.
3--It will ask you if you want a Software, Real Instrument, or Electric Guitar track, select Software and select the Create button in the bottom right of the window.
4--Now you should see a new track load into the GarageBand window, and it should say Grand Piano.
5--Now on the right side of the window you will see a picture of the piano and just under the picture are 2 tabs, Browse and Edit, select the Edit button.
6--Now you should see an option that says Sound Generator with a picture of the piano keyboard, just to the right of that are 2 drop down menus that say Piano and Manual.
7--Select the Piano drop down and it will give you a huge list of GarageBand Modules and Audio Unit Modules, you want to look at the Audio Unit Modules.
8--You should see EZplayer and EZdrummer as a list item, (I am using Superior Drummer so that is listed for me), it should show all VST's that you have installed.
9--Select EZdrummer and the picture should change from the piano keyboard to an orange circle with some curved lines on both sides.
10--Click the picture of the circle with the lines and EZdrummer should open up for you showing your drum kit. Now that track is set for EZdrummer.
11--Now you should be able to hit your ZX and it should sound the drums assigned to those MIDI numbers.
12--Now you can make changes to your kit when the EZdrummer window is open, you can change snare drums, MIDI note assignments and so on.
Another thing to look at if you are still not hearing any sound, at the bottom of the GarageBand window is a series of play and reverse buttons, right next to that is a blue window that shows measures and has some numbers in it, when you hit your ZX, you should see a tiny blue dot come on every time a MIDI signal is received, if you don't see it, then GB is not getting your MIDI signal. If that is the case, it will have something to do with your MIDI to USB device, if it still happens, you may want to just go and get an UNO, they are less than $50 I think at Guitar Center, I have two that I use, and that should fix your problem for sure.
Hope this helps.