Are you using programming software? For your scanner you can use ARC500 or PSREdit500. I use Butel ARC500 Basic to program my Radio Shack PRO-106, the older brother of your scanner.
On the channel you have for Polk SO, press EDIT, then CURR, and then scroll down to Search Type, press right until you end up at P25, then go down to the NAC entry below it, enter 490, and save it.