You start the level inside the docking tube, which has been destroyed such that its artificial gravity isn't working. If you have Tali, be sure to upgrade her Powers in the setup screen - especially Sabotage and Combat Drone.

There are three crosshairs around the star, but the Dreadnought is the one between the star and the first planet, Rannoch. The Geth Dreadnought is in the Tikkun system, the only system you can visit in the Perseus Veil.