I am trying to unlock my email and the site said it sent me a key but my email is not receiving emails either.

If the activation key has been resent, but you cannot receive new messages in eM Client, then you will have to use the web interface for your email account to view the message. If it is not in the Inbox, maybe look in the spam folder.

Once you have it just copy the activation key from your web browser and paste in in eM Client in Menu > Help > License > Activate.