I'm trying to do the quest for Scar to find ut why people have been going missing. I searched the sewers and found the ogre mage and all the carrion crawlers, after killing them I returned to Scar to tell him what i had found but wasn't given the option to tell him. Do I need to pick something up or have something to give him in order to tell him?
Also, Gervise asked me to to find and kill a druid called Valten who was plannning to polymorph herself and kidnap two little girls. But where can I find Valten?

Try talking to the girls in question.

Also, try searching on some of the statues on the wall beside the ogre, you might find something to comfort the families of the deceased ;)

KILL EVERYTHING IN SIGHT!!!

Oh, wait...that won't work...scratch that....

I don't know about the missing people...when I did that quest, I simply talked to Scar, talked to the ogre mage, got the "journal updated" notification, and then went back to Scar...

Just in case, check your journal!