If you're speaking of centering it in relation to its Label, you'll have to Delete the Access generated, attached Label and create your own, then align the Checkbox and Label as you want it.
Separating the attached Label from the Checkbox itself can be tricky, but you'll just have to try selecting and deleting it. Try clicking on the top, right hand corner of the Control and delete it; this should leave the Checkbox by itself. Then simply add an independent Label and position the two as you want.