The 5V Antenna Power function is for use with a masthead amplifier on your antenna. If you don't have a masthead amplifier this function 'should' not help (having said that, with antennas I would believe anything). Most masthead amps in Australia are not 5V so a separate power source must be used if you have one.
The system works by sending voltage up the antenna cable core and returning it via the shielding. The amplifier picks up the voltage and also isolates it from the antenna. If you don't have an amplifier the voltage would be directly across the antenna which is not a good idea and probably why you get an error message.
So, you have a signal problem but unless you have a 5V masthead amp (unlikely) you should not use the 5V function. Do you get good signal with antenna directly to the TV?
UPDATE: Thinking about it some more, maybe you DO have a masthead amp (which could be anything from 12V upwards) and by some miracle the 5V power from the Humax is enough to get it working, but the current draw due to the voltage mis-match is too much for the Humax to sustain.