I presume you're simply looking for the area of the square and you are only given that the diameter of the circle is = 2. You know that the circle circumscribes the square and thus the hypotenuse = the diameter.
You also know the relationship between the isosceles triangle is 1, 1, sqrt(2).
You need to find the side length. So, because you know that diameter = 2 and that hypotenuse of that special triangle = sqrt(2), you can easily calculate the side length by simply finding the proportion. which would be 2/sqrt(2).
then you square that to get the area. if you need more explanation let me know. Not sure if that was helpful.