Hey DUDE from MD with big hole in DJX....
What you are trying to accomplish is so damn simple.
First..... assuming your keyboard has only ONE audio output, the Phones/AUX out, and that that output is 1/4 inch (big hole). The HOLE will be either 1/4 inch or 1/8 inch anyway, just measure the diameter with a ruler. Forget about the output labelled MIDI OUT for now, that's for connecting other midi instruments or for connecting your keyboard to a computer. Second assumption..... your cassette player has 2 input terminals (probably on the back side), one for left channel and one for right, same kind of jacks most stereo systems have (looks like 2 nipples with a hole in the center of each).
What you need.... go to a Radio Shack and get a wire with a 1/8 inch stereo plug on one end and 2 RCA plugs on the other end. This is a standard wire used for connecting things like Walkmans and Discmans to a home stereo system. If the output on the keyboard was 1/4 inch, also buy an adapter that goes from 1/8 to 1/4. Total cost should only be about $5.
How to..... attach the 2 RCA plugs to the left and right channel inputs on your cassette deck. Plug the 1/8 inch end into the adapter. Plug the adapter into your big hole phones/aux out on the keyboard.
You are connected and ready to record. Amazingly simple.
There still is the possiblity that your cassette deck has a different type of input but you will have to be more specific in describing it or just take the deck to a Radio Shack and they will tell you where the input is.