There are two separate steps for trunked systems.
If you are setting up the trunking system itself, you enter the frequencies listed at the top of the page for the site.
If you are setting up the talkgroup lists / subgroups, then yes you'll use the DEC # which is the talkgroup ID.
If you want to be able to listen to *anything* that happens on the system, you don't need to plug in TGIDs, unless you want to set up alpha-tags.