Most likely the email we sent you is in the “Spam” or “Junk folder”. Depending on the algorithm used by the email service provider in your case, this is done to protect you from spam.
In this case, check the “Spam” folder, find the email we sent you, and mark it as “Not Spam”. The email will be moved to your Inbox. Then use it to confirm your email address and then log in with the temporary password we sent you.
To avoid being considered Spam an email that we will send you in the future, see here.
Another option is you used another (valid) email address, but not your own email. For example, instead of giannopoulos@gmail.com, you used (by mistake) something like ginnopoulos@gmail.com.
In this case, repeat your registration.