It's not clear exactly what you're calling an "element". I'll use more precise terminology. If your Iregion structure array is just one structure, then you have just one blob. The Centroid field of that structure will be a 1-by-2 array with the x and y locations of the centroid of that blob.
If you have multiple blobs, then you need to add an index after the structure variable name to get just one of the blobs, try this:
props = regionprops(bw2,'centroid');
for k = 1 : length(props)
xCentroid = props(k).Centroid(1);
yCentroid = props(k).Centroid(2);
plot(xCentroid, yCentroid, 'r+', 'MarkerSize', 40, 'LineWidth', 2);
message = sprintf('The largest blob has area %d and a centroid at (x,y) = (%f, %f)',...
props.Area, xCentroid, yCentroid)
You can also get all the x and y centroids into arrays doing this:
centroids = [props.Centroid];
xCentroids = centroids(1:2:end);
yCentroids = centroids(2:2:end);