Same as a normal suduko but it's round ! So you have to have the numbers 1-9 in each space ,in the rings going round the circle and 1-9 in the spaces in the lines going into middle of the the circle .The radial ones .
In other words you fill in grid the so that every ring and every circle segment contains the digits 1 - 9.
Hard to explain on here but I know the one you mean ..if you can't do it they give the solutions so you can check here
http://www.bananas.me.uk/comping/marin.html