'disk image fails to mount' - help!
 

Hi guys,
I'm stumped with this particular poser, so hope someone can put me out of my misery. When I try to open this download (it's an extras pack with the extension '.dmg', which I understand to mean disk image, I get an warning dialogue box telling me that the "following disk images failed to mount", then lists the reason for this as "no mountable file systems". I've tried opening my disk utility program and mounting an image for the darn thing, but it doesn't seem to work for me. Any ideas? I'm using an Emac. Thanx in advance.