1. Wrong email address
It's easy to overlook a spelling mistake. It might help to check that your adviser has registered you under the correct email address.
2. Spam folder
Sometimes mail gets caught in the spam folder. Check it to see if your access-code made its way there. Note that some email service providers call this the "junk folder".
3. Corporate spam filter
If your email address belongs to your work or a corporate body of sorts, you might want to check with your IT support team or System Administrator, if it is potentially catching our access-code email.
4. Full inbox
This one is rare but it can happen. If your email inbox is full, new mail will be rejected. Make sure it has enough room for new mail to come through. If your inbox was full, you can have your access-code resent.
Usually email takes up to a few minutes to arrive. Sometimes, however, it may take several days. Other times it may fail and nobody is informed. If you find yourself in such a situation, it could help to have the email resent.