I did some research on your Radio Shack LK-1500 (42-4069) USB MIDI keyboard at both Radio Shack's and Casio's websites. Unfortunately I think your keyboard may only work with a PC through the USB cable and not with a Mac. I could find no downloadable MIDI drivers at either website for Mac.
There is something you can try. Plug your keyboard's USB device cable into one of your Mac's USB ports. Launch GarageBand, pull down the GarageBand menu next to the Apple menu, and select Preferences, then mouseclick Audio/MIDI. Look for MIDI Status: (# of) MIDI Input(s) detected. If it says zero (0) MIDI Inputs detected, then GarageBand is not recognizing your MIDI keyboard.
There is also something else you can try. Double click your Macintosh HD. Open the Applications folder. Open the Utilities folder. Click the Audio MIDI Setup. Click the MIDI Devices button. At the top of your desktop screen select Help and read about Audio MIDI Setup. It should tell you how to add a MIDI device.
If none of this works, then to use MIDI you would need to get a MIDI keyboard that is compatible with Mac OS X.
Remember that MIDI in/out (I/O) is not an audio I/O. MIDI receives/records/sequences/transmits the performance, how hard you hit a key, how long you hold a note, etc., so there is no sound with a MIDI connection.
However, the LK-1500 has an audio out jack on the back. You can use that to connect the audio output of your keyboard to your Mac audio in port if the keyboard has a line level output. You will need a mono 1/4" plug to 3.5mm plug cable. Radio Shack should have one. If the keyboard audio output is not line level, then you need a USB audio interface between the keyboard and your Mac.
