I have the same lens in 180mm. The two discs are noted by the color of the dot. Basically, they reduce the amount of light, which affects the aperture and also affects the degree of softness in the final image. If you look at the f-stop markings, there is a difference in the scale for each lens disc, which is shown by the color on the scale (it corresponds to the disc).
The disc with the small center opening has a softer look than the disc with the larger center. Do a few exposures to see which one looks best to you and go from there. They take a bit of getting used to, but work well.