In walkthrough says that the sewage golem (irecnius dungeon, begining of the game) should open the gate to prison cells after you tell him to. But I get the mesage something about he being busy and unable to talk. How to get him open the door? How to get into next level in the first place? Help me avoid cheating, please?

This is a well-known bug. Install the official patch from Bioware. (Install the SoA one if you only have SoA; install the ToB one ONLY if you have ToB installed.) The link to the patches is in Icelus' mod sticky thread at the top of the forum page.