You need Disk Copy (included with all Apple systems):

1. Type command F and search for it in Sherlock

2. Double click its icon

3. Drag the .dmg file into the Disk Copy window and it should mount on the desktop. If it doesn't mount, it may be corrupt (unfortunately many files on Gnutella are).

NOTE: Although .dmg will mount in previous OSs , it is a Mac OS X format so you will need to boot up in OSX to install it. If you don't have OS X it will almost certainly be useless to you. To confirm this, open the mounted image and you will probably see a file with a .pkg suffix. This is the format for the OSX system installer.

If you only have OS9 or earlier, you should be looking for .sit (open with Stuffit) and .img (open with Disk Copy) files.