If you can preview the HIT but didn't accept it, there on the top left, it says HIT Details in blue font. Click on that and the pop up will bring up "contact requester" on the bottom left. Same thing goes for HITs in your queue as well. Once you contact the requester through Mturk, Mturk will give them your email address so they can reply to you directly. There is a chance that they won't reply. Nothing you can do in those cases unless if you can get their personal email from the consent form at the beginning where they describe what the HIT is about, how to contact them and who is organizing it. They are more likely to respond to those personal emails.