First of all I do not own this keyboard. But a quick look at the manual (found on the internet) says that when first powered up the SP88 is in Internal Sounds mode. Meaning you hear the sounds selected when playing the keyboard. It also stated that the default Midi Transmit Channel is Midi Channel #1.
Looks like you would need to go to the SP88 Keyboard's MIDI SETUP and simply select LOCAL OFF. Now the keyboard should be ready to transmit Midi Out only and no internal sounds should be heard.
Next you may or may not need to do this but you might need to go into the Utilities Folder found in your Applications Folder on your Mac and double click AUDIO MIDI SETUP the keyboard Icon. Figure #1 below.
Next you need to make a physical connection from the Midi Out of the SP88 to the Midi In on your Midi Interface. Figure #3 below. You may need to Click on Add Device first. Figure #2 below.
In the examples shown below I have a Yamaha Electronic Drum Kit that is Midi Out only... not USB out. I had to plug a Midi cable from the Midi Out Port on the back of my Drum Module to the Midi In Port on the back of my Motu 828 Audio Inferface which is connected to my Mac. Next I created the Audio Interface and Drum Module "Devices" as mentioned above.
I then double clicked on the Drum Module and made my Midi Out Selections. Figure #4 Below. And once it was setup and the icons closed I used my mouse to draw a virtual cable connection from the Drum Module Out to the Audio Interface In. Figure #3 below. This is how I got my Midi Devices to talk to one another.
Once you have made your setup as I have outlined create a MIDI Instrument Track in Garageband, select an instrument for example a Piano then Record Enable that track in Garageband and see if you now hear what you are playing. In this example the SP88 is a Midi Controller only and you will only hear sounds selected in Garageband which is what you want.
Keep in midi your setup may be different than mine. You can keep your SP88 Keyboard on the default Midi Channel #1. That would be a good place to start. Garageband receives Midi on all channels.
