Checkbox type Custom fields is not possible to make mandatory. However, there is a possibility to make some Standard fields Checkbox Type mandatory. When customizing entry form, it is possible to check Mandatory Checkbox related to Standard Checkbox field on the Fields subtab, e.g. Available to Adv. Partner field.
Note: Setting Checkbox mandatory does not mean, that it has to be checked. Both T or F statuses of the Checkbox let user to save the record.