How can I add value to my Octopus card directly from my debit/credit card in Hong Kong?
The couple of "Add Value Machine" I have seen in the MTR and HKG airport only accepts cash:
Simple answer - You can't. Cash only! This is true not only at the Add Value Machines, but also at the service desk and all other locations.
Longer Answer - It is actually possible to top-up an Octopus card using a credit card, but it can only be done using the "Automatic Add Value Service" (mirror), which is only available for customers of specific Hong Kong banks. Presuming that you are not a HK/Chinese resident, then this option is not available to you, so we're back to the Simple answer of "You Can't"