I-H616 500W Inline External Heater

I have only gotten a flashing "Lo" since plugging it in. What am I missing from the limited instructions?

Which wattage should I use for a 66 gallon aquarium?

can i hook this up to a eheim 1500xl?

Is there a reason as to why the inlet/outlet ports have changed? I ordered the 500w 16/22, but seems to have both sizes on the ports with no lock nuts