I want to know how to get my mac to recognize and read .dmg files.

Upgrade to OS X. .dmg's are only used on OS X, not OS 9.

I can't Its a powermac 5250 and it can't even handle in any way OS X.

There are ways, but it depends on the DMG image. The last official Disk Copy release for OS 9, v6.3.3, won't open any DMG images. However, you can get hold of the newer Disk Copy 6.4 which was only made available to developers. This will open certain types of DMG image. DMG images are like normal IMG files in as much as they can either be compressed or uncompressed. What's catches people out on OS 9 is that compressed DMG images cannot be opened on that system, not even by the newer Disk Copy. It's unfortunet but there's no way round. There is an urban myth that states if you use the even newer Disk Copy 6.5 beta 11/13 for OS 9 (which was scrapped when OS 10 went mainstream), that you're supposed to be bale to open even more types of DMG. This is an absolute crock of s---, so dn't believe anything you read about it. The truth is that only uncompressed DMG files can be opend on OS 9 using any of those Disk Copy versions. That's it. Compressed DMG's cannot and never will be openable on OS 9. Fact. You can easily tell if a DMG is compressed. Use some sort of application capable of reading the datafork with hex data (a simple hex editor will do the trick). If the first two digits are 78, the disk is compressed, and you can there and then rule out ever opening it on OS 9. If it isn't compressed, you will need Disk Copy 6.4 or, 6.5b11/6.5b13 in order to open. I don't think it matters which one you use. Again the myth suggests the betas are capable of opening a wider range, but yet again, there is no 'wider range', only compressed or uncompressed. And only uncompressed DMGs can be opened by those Disk Copy versions. Do a search on google for them. You mgith also want to check for a hex editor if you don't already have one and check the DMG contents for those two b---tard digits '78'.