On such short notice, your best bet will be updating the database and using zip code scanning. To program manually, it looks like Quin is the best site in Pike County for McComb. Under the Site Options tab, Trunking Tables must be set to Auto Fill since the site uses 700 MHz frequencies. You only need to enter the 3 red control channel capable frequencies. Drop the "c" when entering the frequencies.
1 (1) | 028 (1C) | Quin | Pike | 769.15625 | 769.21875 | 769.40625c | 769.65625c | 769.90625c | 773.48125 | 773.93125 |