How do you unlock this quest for Chapter 3, if you are on Ivoerth's path?
In chapter II you find an encrypted note in Malgeth's house (no quest will lead you there on Iorveth's path but if you know the location you can get the note).On Roche's path you will get there with quest "little sister"(something like that) after you can either choose to sell it to Dethmold or to keep it for later use. If you keep it, the quest continues.
Chapter III Bras of Ban Ard can decipher the note, thus giving you the password you will need later. Than you need to find the entrance to the sewers, located near The Mighty Numa. Shortly after entering the sewers you will encounter the Wisp, asking for the password.
Links and more info http://witcher.wikia.com/wiki/From_a_Bygone_Era
Some description of malgeth's house location(can't do better since I wasn't there myself)
"When you cross the fog, nothing stops you from going to the house on the hill, blowing away the barrels, lighting up the candles, and grabbing the manuscript"