First, in order to realize a "bad debt" expense, you must be an accrual basis taxpayer. If you are recognizing income at the time a service is provided, or goods are sold (as opposed to when you receive payment) you are an accrual based tax payer. A bad debt expense can be taken only against income previously recorded, but for which payment has not been received. If you are a cash basis taxpayer, you probably can't take a bad debt expense. As a cash basis taxpayer, if someone owes you for goods or services, but does not pay you, you have not yet realized any income, so you can't take a bad debt expense against the income you did not receive.
If you are an accrual basis taxpayer, you would report bad debt expense in "Other Miscellaneous Expenses" section.