Witcher 2, The - Assassins of Kings Cheats
The sewers entrance is located next to the arm wrestler in the main part of the city, he uses power enhancing potions to win.. More specificly the entrance is to the left of him and looks like a normal door.
Upon entering the sewers a whisp will ask for the password, here is the exact correct order it goes in..
Zi Uddu-ya Ia Ia Gat Exa Nibbit Kanpa Gat Uddu-zi
I hope this helped =]