What do you need value from mesh grid, you can calculate the points on the perimeter of the circles using pol2cart(). For example
r = 1
[x,y] = pol2cart(linspace(0, 2*pi, 100), r)
will give you x, y coordinates of points in the circle of radius r.
Edit: If you want to find the points from the mesh grid, then the following code can help
x = -5:1:5;
y = -5:1:5;
[X Y] = meshgrid(x,y);
r = 3; % radius
tolerane = 2;
indicator = abs(X.^2 + Y.^2 - r^2) < tolerane;
xCircle = X(indicator);
yCircle = Y(indicator);
Since points on the grid are discrete you will need to define a tolerance to for accepting a point. You will need to tune the tolerance parameter according to the resolution of your grid to get all points on the circle.
