I don't have a Uniden scanner so I can't help you navigate the menus. But, assuming you can do that, step by step programming of a P25 is described on the following page.
There are only 3 required steps.
1. Create a system. There are no required properties. You can use the defaults, except that, if you're not going to program talkgroups, you should make sure that 'ID scan/search' is set to 'search.'
2. Create a site. Once again, there are no required properties.
3. Create at least one frequency for the site.
I'd suggest 855.63750 which is the St. Louis North control channel.
That should be all the programming you need to do. If you enable the system and enter default scan mode, you should recieve all talk groups: police, fire, EMS, the city jail, sheriff, street dept., water dept., etc. (in a Radio Shack / GRE scanners, it's called scanning a wildcard talkgroup).
That's my suggestion, for what it's worth. I hope it helps. You can refine the programming (add alternate control channels, program talkgroups) later.