I highly doubt you operate your rental property using the accrual method. (But you could, and it would be a bit rare if you did).
Basically, you do not report unpaid rent anywhere on any tax return. You can not deduct from your taxable income, that which you were never paid and therefore was never taxed on in the first place.
All legal expenses paid in connection with the eviction and/or collection of back rent are claimed on the SCH E in the rental expenses section, sub-section for Legal and Professional Fees. Those costs will be on line 10 of the SCH E.
