It's under the system finder ...
From the main menu, type "F" for [F]ind; type "S" for ystem; press TAB key once; select type of system (eg. "Motorola"), press TAB key; type in the system ID; press TAB key; next to "RR DB User Id" - type your user id; press TAB again; and type your password.
If all looks correct, press "F8" and wait a few seconds.
Limitations:
You must of course be online for this to work. It may not work with some web proxies. This really only works for Motorola systems. Lindsay likes using "Site 1" for non-networked systems (like Type I and Smartnet) so RR DB channels are not downloaded properly - but talkgroups should come through just fine.
-rick