I assume that you talek about sending MIDI data from your keyboard to GarageBand and not audio.
Check the Audio MIDI Setup utility (Applications folder ➤ Utilities folder) if your computer receives the MIDI input. FIrst of all the Device should be listed in the MIDI Studio window. Select "Test Setup". Now when you send any MIDI information (play a key) on that MIDI controller, an acoustic beep will indicatethe receiving MIDI signal.

If that is ok, then GarageBand should receive that MIDI signal too. Unfortunately, the GarageBand Preferences window only tells you how many Devices are connected but not which one.
Check your COntrol Bar Display in the GarageBand Window when you play your keyboard.
- It displays a dot when you play single notes
- It sdsplays the chord if you play a chord
If you see that, then you know that GarageBand is receiving the MIDI message from your keyboard.
Create a MIDI Track from the New Track Dialog and you should see (on the Volume SLider Meter) and also hear what you play throught the Instrument that is selected by the current Patch.

You are sending a MIDI signal to GarageBand and GarageBand sends that MIDI signal to the Software Instrument that is currently loaded (through the selected Patch) on your selected MIDI Track.
Hope that helps
Edgar Rothermich
http://DingDingMusic.com/Manuals/
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