Not all Suitcase models have the 1/4" jacks on the front (your piano is probably from the early 1970's, before this feature was added). Here's another way to do it:
1. Remove the piano's plastic harp cover.
2. Unplug the RCA cable from the pickup rail.
3. Use a cable with an RCA-male jack on one end and a 1/4"-male jack on the other, connecting the pickup rail's output to the input of the effect you want to use.
4. Use a cable with a 1/4"-male jack on one end and an RCA-female jack on the other, connecting the output of the effect to the piano's internal RCA cable.
Let me know if this makes sense. You'll need to leave the harp cover off while you play, which isn't a problem unless you normally put another keyboard on top of the piano. BTW, does the amplifier itself have any 1/4" jacks on the side, or just the 4-pin input?