First, it depends on the type of trunked system you are trying to monitor. Is it MOtorola, EDACS or LTR? Next, are you receiving the control channel? Note that all you need programmed for MO is the active control channel (ultimately you'll want all the CCs, but just to get started it only takes 1). With the scanner in MAN mode on that channel you should see something like MO: CNTRL xx% on the 4th line. If the value is below 50% the reception is probably not good enough to trunk track. But if it is good, then press SCAN (and you only need that one frequency programmed and not locked out) with the bank set to OPEN and you should receive stuff. Note that when scanning if the bank is set to open you'll see "+" under the bank number on the display.
Some things to read include the WIKI section. Search for trunking.
Also click on DATABASE and find the system for your area. What type is it? It is analog or digital? If digital the 2055 can't receive it.