After you have downloaded and unzipped the client and made sure java is installed, your next step will be to log in to the server. Make sure no firewalls are blocking the program at proceed to run the StartClient.Bat that is in the client folder. This will cause a login interface to pop up that should look something like this (right).
The first time logging into the server you must leave the password field blank. For a username use whatever appropriate handle you wish, though once in the server you may have your name changed changed to a faction appropriate name by a server admin. (The campaign has a tradition of players adopting Regimental or Cluster names as player names.)
Once you have logged in (If you are prompted to log in a second time after the login process starts the client may have been performing an auto-update) you will find yourself at a screen prompting you to "get started" after clicking on the prompt you will be asked to register your name and come up with a password. It is a good idea to make this password something you will easily remember as the retrieval process for an account with a forgotten password is very difficult.
After you have completed the registration process you will find yourself in "Boot Camp". Some servers require you to play a certain number of practice games or training missions before you can enter the campaign. We figure that if you have come this far you probably know how to play BattleTech and thus "Boot Camp" is simply a portal through which you choose your faction.
To choose the faction you wish to join simply click on the "Campaign" Tab at the top of your client and select "other" on the menu and then "defect" on the sub-menu. This will call up a pop-up menu that will allow you to select the available faction that you wish to defect to. After selecting a faction you will be prompted on your main screen, in red letters, to confirm that you actually want to defect to this faction. Confirm your selection and you have officially entered the campaign!

On arrival to your chosen faction, you should promptly speak to your faction's moderator (designated by the a ^ symbol net to their name), a campaign moderator (designated by the a * symbol net to their name), or a campaign administrator (designated by a @ symbol next to their name. You will need to speak to one of these before you can proceed further in the campaign. They will ask you to choose an appropriate canon Regiment, Division or Cluster to represent as well as assigning you to your desired sub-faction (Not all factions have multiple sub-factions.) This is a requirement for participating in the campaign.