Pro sound speakers are often equipped with two 1/4" jacks wired in parallel. One is for the signal from the amp, and the other is to attach a speaker cable for another speaker. If you do not use an additional speaker then nothing is connected to the second jack.
Using regular speaker cable, wire the positive side of the cable to the tip, and the negative side to the sleeve on your 1/4" plug.
Using regular speaker cable, wire the positive side of the cable to the tip, and the negative side to the sleeve on your 1/4" plug.