Those outputs are actually mono. When combined they provide the desired stereo effect
You can use the cable to connect the L or R output to the speaker, yet need to be careful when using panning on the channel buses as this will effect volume in this situation
If you only have one speaker, you will only be able to output mono sound. Stereo sound requires two speakers. By convention, you should use the left 1/4 in. output to run to your speaker and ensure that the PAN knobs are all moved to the left (since you aren't using the right output). It is also worth noting that unless your speaker is powered, you will ...
You are not compelled to create a stereo mix if you can handle only a monaural output. Simply use the "PAN" controls on the inputs to send everything to the LEFT (or CENTER, etc.) and use just the LEFT output.
It is NOT recommended to use a Y-cable to combine the audio from several outputs. The guy at the music store was correct.