Hi, Kat - First you have to set up your keyboard to play on your computer. I'm hoping your keyboard will have instructions, but it could be that all you have to do is plug it into the computer with a MIDI cable (with the keyboard and computer turned off), and then turn the keyboard on first and then the computer. Windows may be able to recognize and install the keyboard, or maybe it won't need installation at all - that depends on your MIDI keyboard and on your sound card. If you play it and can hear it on your computer, you'll know it's working.
Within Noteworthy Composer, select Tools/Options, and then select the "record" tab. The "input device" block should have a list of input devices installed on your computer. Select one, and give it a try.
I don't have a MIDI cable, so I haven't tried this. This should give you a start, though.
Good luck.
-Joe-