You are talking about the dispersion option inside the Maxwell Material Editor - am I right? (and not the "dispersion" in the plugin render settings - please check by the way that it is ON too - it should be because this is the default).
Dispersion depends on the material settings, so please make sure those are correct.
Please keep in mind that dispersion usually needs high SL level to start to appear on the image.
I have changed the dispersion flag in the MAxwell MAterial Editor, and it was ok when I reloaded the file. I use the latest "Early Build" version from Maxwell.