Having trouble downloading

[Judy]

Hello,
 
After reading an article I am interested in trying out this program. I don't clearly understand what it is that I need to do to download Gnucash. I went ahead and downloaded Gnucash-Intel-2.3.7.dmg, however, when I try and click on the file my computer does not recognize the program//software. From reading the front page on your website I understand there's another program that I need in order to be able to open this one. Where should I get those programs from?
 
I would be happy if someone could provide me with some clear instructions on what I need to do to get this program running on my Window Vista PC. I'm looking forward to trying it out.
 
Thank you,
 
Judy