On your Wii's Wii Shop Channel, you should be able to find an option that let's you purchase Wii Points via Credit Card. I'd tell you precisely where, but I can't access my Wii at the moment, being it's broken... Sorry.
for one thing the ZIP code in Singapore is 6 digits but the Wii Shop channel only accepts 5 digits and it does recognize that it's not a valid US ZIP codes.