Open up Mechanical Turk, sign in if you haven't already, and copy and paste the following to your address bar (replacing www.mturk.com/mturk/dashboard) and press the Enter key: A window will pop up telling you your worker ID.
It used to work ... it doesn't anymore. You now apparently have to contact the MT folks, but they don't seem to answer.
I know this isn't the correct way to find your ID, but if you have ever contacted a requester and received a reply, your ID will be there in that email. Check it out.
Just like magic, one day I was looking at my account and noticed that at the right hand upper corner was......my ID. At least, I assume those 14 alpha/numeric characters constitute my Worker ID. Maybe it's even labeled. I don't remember. I don't know whether it was always there and I just noticed or what. TAMedia had sent me a message earlier in the week, full of ID numbers and I didn't know what mine was or where to find it. Maybe it is just that simple. Upper right hand corner..........?