Use the Galaxy menu to select which galaxy to model. Then you can adjust the various sliders to modify the halo (dark matter) central density, the halo scale length, and the mass-to-light ratio so that the red line (which represents the orbital speed as a function of radius predicted by the combination of the disk and halo models) most closely matches the experimental data in blue. The red Chi Squared value will be minimized for the best possible match.
The orange curve shows the speed as a function of radius that we would expect from the dark matter alone, and the green curve shows the speed that we would expect from the stars in the disk alone. To estimate the ratio of halo (dark matter) mass to disk (star) mass within a given radius, find the red curve that best matches the experimental data, then calculate the square of the ratio of the speed due to the halo to that due to the disk at that given radius. See Chris Mihos' RotCurve website for more details about the physics.