I know this is an old post, but this might help someone.
I have never found a way to centre align a check-box in ms access when used in a tabular layout.
However, there are work arounds. If you desperately want to have the check-box in tabular layout then the only way to do this is a as follows.
Create a button and add this button to the left of the check-box so it is also in tabular layout.
Remove any caption on the button. This will add space to the left of the check-box. You can then resize the button to make the check-box centred between itself and the new button.
If you have borders on your control labels you will have to remove the borders on the check-box label and the new button label by setting the border style to transparent.
Then create a new label, with a border if you wish, which spans over the button and the check-box which you can name to the original caption of your check-box's label.
I hope that makes sense, good luck.