If you hold an account, you can log into the system. As a user of the system, you may have been granted permission to view specific datasets at a higher resolution than a public user (this will depend on the agreement between yourself and the National Biodiversity Data Centre).
To login, simply enter your Username and Password and click on the OK button. If you have not registered for an account then you can do so by clicking on the blue 'Register now' link. This link will open a dialog window where you can enter your details and register for a new account.
Once you have logged in successfully, you will notice that the 'Login' link in the top-right corner has now changed to 'Logout'. To Logout simply click once on this link. Also note that your user name is displayed to the left of the Logout link. If you click on your user name you will be redirected to a page where you can modify your profile.