Installer on OS X 'Disk Image Won't Mount' error
If you are getting an error message telling you the "disk image couldn't be opened", you might have an incomplete download. The Lemur Installer is 7.4MB, try downloading it again. If the file is complete, mounting DMGs in general has been reported as an issue occurring sometimes with the Snow Leopard version of OS X. Try mounting the DMG with Disk Utility, found in /Applications/Utilities.