If you saw the cutscene with Cero Falk, the door should open. The entrance is in the same map as the Spider's Nest, do you have the right building?
34 days ago(+1)
And I am sorry for giving you incorrect information last time. I checked to make sure, and the questlog tells you what you should do before trying to enter the sewers: you must go to a library to do a research about the catacombs first. For some reason, I forgot about that (I didn't even mention it in the walkthrough, so I'll fix that as soon as possible).
On the plus side, you should be able to finish the quest without issues, now.
28 days ago(+1)
There are various libraries in the game: The closest one in Tarkas is accessible if you are part of the mage guild. Go to the library in the basement and do your research there (there is a shining spot in the library). If you are a student at the magic academy, there is also a library there (ground floor, top-left corner). There are others, but these are probably the easiest ones to access. I might have to add one you can access just by paying a fee, for those that don't want to progress any of the secondary paths.