There is no easy means to convert the frequency of your line power. Changing voltage is easy (step-up or step-down transformer), but to change frequency you need to literally "regenerate" the power. Fortunately, that's exactly what the PS Audio products like the P300 do. (And they can be intrenally switched to run off 220v).
However, I've never heard of an electronics product like a processor that wouldn't run fine on either 50 or 60 Hz. I just moved back from England and ran all my AV gear on 50 cycle power with no trouble. I suspect if you contact Krell, they might tell you to ignore the error message and press on. Does the unit seem to function OK, other than the error message?