I have the same scanner and my best advice would be to download and learn how to use FreeScan. If you know how to cut and paste there's an EZ Grab function within FreeScan that will make it very simple to add conventional frequencies. Trunking is a bit more difficult, so read the referenced Wiki. You're stuck using the online manual or the Easier to Read manual for the BCT15.
Easier to Read BCT-15 Scanner Manual The BCT15x is the update to the BCT15, so they are similar enough that this online source should help.
Ask lots of questions and read up on others having trouble. There's no lack of that on here!.....
All of the DMA Uniden scanners share the basics for programming, so the 346xt, 396xt, 996xt should give you some helpful info. Written manual or the ability to print an online manual seem to be the biggest complaints, but Uniden has turned a deaf ear to that. Online video tutorial would be a big addition for new comers also, but hasn't happened yet. There is a trunking tutorial on the Uniden website that gives you the basics of trunking systems, but stops at that, so doesn't give you any programming help.
There's information out there, but spread all over the place, so be patient and try not to get too frustrated. Take it one step at a time it will eventually start to click. Then you can come on here in a month or two after you've forgot the frustration and tell everyone how easy it was!.....