Once that new window opens, simple click on the "Accounts" button and then click on the "+" sign to create a new email account. Once the screen to create a new account appears, simply type in the email address for your Exchange 2007 account and the password. Once that's done, Mail will autodiscover the correct Exchange information (provided your Exchange 2007 server is set up properly for this) and populate the correct information automatically.
During this process you will also be prompted to include Address Book and iCal sync. Allowing this will create a new account in both your Address Book and in iCal which will contain your Exchange information. This new account will show up in the left column of both Address Book and iCal as your Exchange account. This will also allow you to use the Exchange Global Address List as well as your own personal contacts when sending email in Mail.
In addition, you can set events, meetings, appointments, etc. in iCal and invite other users in your organization to attend them. They can then, in turn, respond and confirm or deny attendance or suggest an alternative time or date for the event. So far, this works pretty well, but I haven't had time to test it more thoroughly; I will update this article as I do.
These new features in Snow Leopard are pretty cool and a welcome addition to OS X -- especially for those of us who need to work in a mixed environment. As I get more time to explore these new features of Snow Leopard and Exchange, I'm sure I'll find more things to like about the way they work together -- and probably a few things not to like as well. We've had a report from some early users who are having trouble sending any file attachments over 8 MB in size, and another report of spurious meeting change notifications when canceling recurring meetings; though I haven't experienced these particular issues, please do let us know if you are seeing them or other gotchas with Snow Leopard & Exchange.