<http://www.marcofolio.net/photoshop/create_a_fish_eye_lens_effect_in_photoshop.html>
I prefer, however, to use the Warp transformation, since it has a Fisheye preset. Select the area of the image that you want to make fisheye (select all if you want) then Edit > Transform > Warp. In the Tool Options bar, select Fisheye as Warp type. You can adjust the amount of warping by dragging the control point up or down.