How to mount.dmg image file?

I'm trying to install ios 7 beta 6 but instructions say:
1) Mount the.dmg image file;
2) open iTunes;
3) connect the iPhone/iPad/iPod to the Mac/PC with the USB cable;
4) select Restore while keeping pressed Alt (Mac) or Shift (Windows);
5) choose the.ipsw firmware file that is located within the.dmg mounted image;
6) once restored, go here to activate: http://www.incredibili.net/iOS7

The first step I don't understand

There's no beta. Apple never offers any beta. You have pirated software. Apple requires that you pay $99 for a developer membership to get a developer version of iOS 7, and you didn't pay, so you are doing something illegal to avoid paying.

To mount a dmg file just double click it (.dmg us just for mac)

You would need to download an IPSW file if you're on Windows, not a DMG. Also, it's completely unsafe to install the beta unless you register your device's UDID with a developer account through a site like http://www.udid.in/.It's $5, and then you don't need to worry about Apple deactivating your device for running unreleased software.

If you are on Windows, you have to open the.dmg with something like PCDmg ( http://www.acutesystems.com/scrpd.htm ) and drag out the.ipsw file