Exchange 2000 and NTBackup util

I have a box with Windows 2000 Server SP4 and Exchange 2000 SP3. I want to backup individual items in the users mailboxes. I have a Sybex book, “Mastering Exchange 2000 server” that says I can browse the M:\ drive in ntbackup. When I try it shows no items in users mailboxes. What do I need to do to see items in users mailboxes?
I really need to know ASAP, hince the high points.
Thanks,
David