Ignore the first two steps in the article, since there's only going to be one account called "System Mailbox." You don't need to find the GUID in ESM.
Open the AD Users and Computers tool. Don't try to use the SBS Server Management tool. Open the distinct tool. Pull down the View menu and choose Advanced Features.
You do everything in the article from ADU&C, not the ESM. When you get properties on the System Mailbox, one of the tabs is titled "Exchange Advanced".
ShackDaddy