switcher dmg file problem

i have a dmg file on a disc, insert the disc, open the disc and then double click the app to open it. but a warning box containing "the following disk images failed to mount" reason - "no mountable file systems" appears.

the file is 200 MB and i am severely confused, iv only had a mac like 2 days n cant seem to do anything on it :S


any help would be greatly appreciated

By double-click the app you mean double click the DMG file right? Anyway, what file is it? Go to disk utility, (applications > utilities) and the drag and drop the DMG file into the lower left pane of Disk Utility. Then click on it and hit verify disk.