From Causeway Bay in HK, can I ride the Dingding to Central instead of MTR? How do I know I've reached

What specific place you are at the Causeway Bay area?
If you want to take the Dingding bus, you can walk to the Paterson Street Station to take the Causeway Bay - Shek Tong Tsui Line to reach the Pedder Street Station Then walk to the Pier to find the bus line 15.

The dingding bus cannot take you to reach the destination directly and you need to have a map to guide you to find the direction. But it is worth trying.