Look Jesus, the fact is that , if the Earth is flat, the Centre of the Earth is the North Pole. A compass points at the North pole, so is constantly pointing towards the centre of the disc.
Consequently, if you go, say, East, you are going at a right angle to North, as you look at a compass. However, as soon as you go a small distance you will have moved to a different point and North will be in a different place. You keep going East, but to do this you have to adjust your direction. In order to do this, you have to go in a circle.
Thats how these guys justify their arguements. Unfortunately for them, there are other ways of travelling in a straight line.