You must have an account with this site, in order to access it. Sign in here, if you have already registered for one.
Otherwise, visit the sign up page for an account. Access to that page is restricted to the PSFC network.
If you are not on the network, that page will prompt you for a username and password. Enter your PSFC credentials.
It is not necessary for you to create an account, if you just want to subscribe to a mailing list. You can sign up via email.
For example, if the list name is plasma, send an empty email to plasma-subscribe@lists.psfc.mit.edu.