The "bull" is on the centre of the dartboard and is worth 50 (fifty) points. There are loads of different ways and combinations to get this achievement but here is a quick way to do it...
When playing a 301 match, score a 180 (Treble 20 x 3) and youre left with 121. Finish it with Treble 20, Single 11 and the Bullseye.
To hit the bull easily, aim at the top wire of the treble 20, right at the center, and throw your dart with no power at all (down and then up on the right stick right away). The dart will land on the bullseye.