Where is the knock sensor located?

Actually, I do not have the answer, I am looking for the knock-sensor myself. I just had the car (1998 Altime SE) looked at today and was told knock sensor AND the vent control valve (evaporitive canister) were both bad. I don't know where either of them are.

Buy the Haynes manual for your car {#72015} at auto store {around $15-18}
It will give you that info and a whole lot more ! The knock sensor is located
Near the oil filter.Illustration on pg. 6-11.

The more important question... why are you looking at the knock sensor? If your 98 is throwing a code pointing you in that direction but the engine runs okay (you're not hearing any knocking, pinging or rough idling), there may be something else causing the issue. The same thing happened on my daughter's 98 altima with 165,000, turned out the air conditioner drain was hopelessly plugged and the condensed water was running down a nearby cable into the computer. The computer board was corroded beyond repair. $1,300 later, the knock sensor problem was solved. The moral of the story - buy the Haynes manual and do a solid diagnosis instead of chasing symptoms.