Leave the page open when you check your email. Copy the code from the email into the box and you'll be signed in. This will cover you for up to 5 15 days at your current location, but you may see it more often if your location appears to change - e.g. different browser browsers or clearing cookies.
See also: About multi-factor authentication
Codes can only be used once and must be used within an hour (from the time they're generated). If you're confident that neither of these are the reason a code hasn't worked for you, please contact your usual support provider.
What if I can't access my email?
You will need to have someone update the email address on your account. Ideally this will be someone from your own organisation as they should know that you are you, but if you have to contact us or your support provider you will need your 4 digit pin from the organisation record.