According to the data sheet, D/C=A0 in I2C mode. So if you wired it high (or pulled it up) the address should be 3D instead of 3C. It sounds like you have the other pins wired correctly as well. SCL = D0 and SDA = D1. It would be nice to have a schematic.
I found that I needed to give this display a reset pulse for it to work. I doubt that wiring it for I2C changes that. Maybe a simple RC circuit will function if you don't have a spare I/O pin for reset.