FAQ: firmware

Here's a frequently asked question that will probably unnecessary soonish when iOS 4.2 for iPad is released (tomorrow?). But still, for clarity's sake...
How do I install the DMG firmware update for the iPad?

I downloaded the iOS 4.2 GM seed from developer.apple.com, which is in the form of a dmg file. When I double click on it, it complains that no appropriate file system is mounted. This happens when I have iTunes running and when it's not running.

When I go into iTunes, it rejects any attempt by me to drag and drop the dmg into the iPad area.
When I go into Xcode, it rejects any attempt by me to provide Xcode with the dmg as an alternative to 3.2.2 in the Organizer window.

I know the new iOS is about to be released but I'm still curious: What's the normal procedure for installing a developers copy of the operating system?

 

Update:
I had a corrupted dmg file. I redownloaded it and it opened just fine.
Inside was the ipsw file.