Check your installed Photoshop files each version of Photoshop you have installed should have its copy of the file. The script is testing to see if it exist. Your does not or its been moved to the wrong place. Use Finder or windows explorer the find all versions of CreateImageStack.jsx on your system. The one for CS5.1 is not where it should be in Photoshop version "Presets\scripts\stack scripts only"
