I have troubled opening file dmg

Dear friends,
Good afternoon, by the way I'm new with majorgeeks.
Well here's the trouble:
I was trying to open a dmg file on my computer (I'm using XP with AMD Turion64), My friend suggesting me to open majorgeeks while I'm browsing, I opened and for the result I have trouble searching on the software for opening the dmg file...
well, I'm new with all this, so please for you guys to kindly let me know on how using the majorgeeks correctly and please tell what should I do for me to open my dmg file.
Thx