Select Manage | Value Lists from the File menu. Use the dialogs found there to create a list of values--the values you want to appear as check boxes in your field.
Go to the layout where you have a field you want to appear with a check boxes format. Enter layout mode. Select the field. On the Inspector's data tab, select a check boxes format and select the name of the value list that you just created as the list of values for your check boxes.