Fight all the Plasma Grunts and beat them. Some of them will give you hints to the password, which is always one of the following:
- 7707
- 2202
- 9909
- Reshiram*
- Zekrom*
*= write it exactly like that, or it won't work.
Also, you may only enter the password into the machine after you receive the Plasma Card from one of the Grunts, which I assume you already have.