First of all, you have to buy Data Pack to activate data package with your credit inside.
Without this, it will incur high usage and your credit will be all used in a second.
To get 7-day Data Pack: HK$48/300MB -> Dial *118*448#
To get 30-day Data Pack: HK$68/500MB -> Dial *118*468#
(Thereafter HK$0.8/MB, Automatically cancelled on the 8th or 31st day 00:00)