Yo Elda!

The .dmg extension means its a disk image file. You should be able to open it by double-clicking on it. Sometimes these appear as text files. If this is what is causing your problem, just download DMG Fixer from http://homepage.mac.com/mnkd/dmgfixer10.sit. Drag the DMG file to the DMG Fixer icon, and it should fix the problems. Now you should be able to open the file just by double-clicking on it.

Hope this helps!