By default, the Security tab is missing in the Exchange System Manager. To display the permissions tab, you have to make a change to the registry.

Here is the correct path to navigate:

HKEY_Current_User\Software\Exchange\ExAdmin.

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: http://support.microsoft.com/default.aspx?scid=kb;en-us;264733