Open Mac Mail and go to "Mail" and "Preferences".
Click "Accounts" at the top and hit the "+" button down the bottom right to add an email account. Select Google account.
Fill in full name, email address and password etc.
After you click "Set Up" click which items of Google Apps you would like to sync with your machine. Common setup for our clients is to select "Mail", "Contacts", and "Calendars" but not "Messages" and "Notes"
Click "Done" and Google Apps should be set up in Mac Mail. For support email us at firstname.lastname@example.org or visit our website.