I spoke to Aran Linvail at the Shadow thief guildhall and he told me to meet up with Mook at the docks at night. When i left it was already dark but MOOK wasn't there...What gives? What do i have to do to find her?

She's literally on the dry dock. Sometimes if it's already night when you get the quest, it won't be triggered until the next night. Wait another day before you see her.