From the "Entourage ReadMe.html" file in the ReadMe folder of the "Microsoft Office X" application folder:
7 Maintaining Your EntourageX Data
7.1 Location of Your Entourage Data.
Entourage stores data such as preferences, rules, schedules, signatures, messages, contacts, and all other Entourage items in a database on your computer. The first time you start Entourage, a new folder, called Microsoft User Data, is created in the Documents folder. Within the Microsoft User Data folder, Entourage creates a folder called OfficeX Identities. This folder contains the Main Identity folder, which stores the data for the first identity. Each time you create a new identity, Entourage adds a folder for it to the OfficeX Identities folder.
7.2 Backing Up Your Entourage Data.
We highly recommend that you make regular backup copies of your data so you can restore it in the unlikely event that your hard disk is damaged or your data becomes unreadable. To make a backup copy of your data, drag the identity folder you wish to back up to another volume, such as a different local or network hard drive or a high-capacity storage disk. To restore the data for an identity, drag the backup copy of the identity folder to the OfficeX Identities folder.