By default, the Security tab is missing in the Exchange System Manager of Exchange 2000 and Exchange 2003. To display the permissions tab, you have to make a change to the registry.
Here is the correct path to navigate:
When you reach the above section of the registry you need to create a DWORD called ShowSecurityPage.
A value of 1 (Numeric one) means on, whilst 0 (Zero) means off.
Close the Exchange System Manger, then reopen (no need for a reboot).
For more info: KB 264733