References won't stay

So I want my game to communicate with an acess DB, and I don’t know nor care weither unity has it’s own way to do it. So I add the System.Data reference to my monoDeveloper, and write the code, but then 2 problems pop up:
First of all, once I leave the project, the System.Data disapears and i have to reinsert it, but the big problem is that unity DOESN’T RECOGNIZE I’M USING THE REFRENCE, which means every time I try to test my game, the program claims the refrence “System.Data” doesnt exist.

Does anyone know why is that?

I would guess it’s because you haven’t imported the System.Data.dll file from the Mono library into your project.