Need help understanding how to use the 240 volt option on my multi tap mh ballast... any help?

You would connect the 240v wire to one leg of your circuit and the common to the other leg. Make sure you properly cap the unused leads and use a double pole switch for safety.

Was thinking that but wasn't sure. Thank you

You're welcome.

This is why the second supply connection is called "common" and not "neutral". It's a neutral for 120V and 277V, but it's a hot for 208V and 240V. The terms "common" and "neutral" are not necessarily the same thing. Think rectangle/square scenario.