Ok, so I've missed alot of the show... well, all of it really... Are they already near earth? the show hasn't been running that long, has it?
They saw a holographic representation of Earth in Home, Part Two in season 2.0.
In the end of season three, a great big "power of ten" zoom in and zoom out showed them to be fairly near Earth.
They'll be getting to Earth in season four, one way or another, this being the last season.