Perform the following steps:
Choose Tools -> Settings.
Click Delegation for Exchange in the left pane.
In the Delegating to Others tabbed section, click Add.
Enter the name of the user in the Delegate To field on the Select User dialog. Alternatively, to select a user from a GAL or a list of contacts, click on the Addressbook button. Use the Select Contacts from Addressbook dialog to select a user.
Click OK on the Select User dialog to assign the user as a delegate.
Click OK on the Settings dialog.