Login accounts are given to bona-fide individuals on request. If you don't have a login you can click on Create new account below the password box. These requests are manually vetted by a site administrator, once authorised you will receive an email with instructions on how to set up a password. You can then login to the site.
You will need to login if you want to contribute (i.e. write articles, upload photos, or add comments.
If you see a login box on the right then you are not logged in, so just enter your username and password.
If you've forgotten your login details, you can click on Request new password. On the page that comes up, enter either your username or email address and you will shortly receive an email that tells you how to create a new password.