Multiple Defaults for Checkboxes

When in the table fields list, you click the field that will be for a checkbox.
The field should be of type checkbox.
Edit the field.
Set Default to True for each checkbox that you want to be selected on first opening.

If however you select the checkboxes type field (plural) then it's a little different.
In the Checkboxes Options section, you create your list of checkboxes with the value and label fields.
In the Default and Value fields at the bottom of the page, you enter the values that you entered for each of the checkboxes above.
eg: if you have values 1 2 and 3 for 3 checkboxes then you enter 1,2,3 in these fields if you want these checkboxes to be selected by default.