The Easier to Read manual has step by step instructions for creating a trunked system.
Easier to Read BCD996XT Digital Scanner Manual
Have you created a trunked system at all? Or just, so far, added conventional frequencies from the database page.
Since you're a Premium subscriber, you could use software, such as
FreeSCAN, which, as the name implies, is free, to download whatever systems you wanted. Much quicker than by hand, and more importantly, you can see most all of the information needed at the same time. That makes it much easier to spot and correct simple errors and omissions made during data entry. If your pc does not have a serial port (labeled as COM 1), see
Connecting scanners via USB - The RadioReference Wiki
The scanner should have come with a serial cable. If you do not have a serial port on the computer, there are a number of USB->Serial converter cables available, or simply get the Uniden USB-1 cable. Links are in the article linked just above.
Also see
Program your DMA Scanner - The RadioReference Wiki