Note: This option will only be visible if your control panel is set to 'Next generation (US English)'.
To enter a delegate, users can select the 'Accounts' tab under 'Settings' in Gmail and click 'Add another account' to enter their delegate's email address.
Once the delegate is signed into their own own Gmail account, they can then access their manager's account from the account selection menu at the top of Gmail. The delegate can access their manager's contacts by clicking the 'Contacts' link. Clicking the 'To', 'Cc', or 'Bcc' links in the mail compose window will also bring up the manager's contacts.