First, you need to sign up for it or receive an invitation from someone (a very good friend). If someone you know has invitations, ask them really nicely if they could send one to you. Otherwise, you will need to sign up for it here: http://imagine-msn.com/minisites/messenger/default.aspx?locale=en-gb
Once you get it, which may take a couple of weeks, you can then download the newest refresh of the BETA (This means that the software is not fully released and is still be tested, which is why only certain people are allowed to use it).
Download it here: http://g.msn.com/1csbeta/Mess80_EN. Remember, you can only sign into this version if you do have an official invitation to use it from Microsoft/Windows Live. Otherwise you must use the 7.5 version.
There have so far been two "refreshes" (this means updates) to the beta itself. The first time it looked a bit weird, with big orange and yellow buttons/borders, and had an annoying effect when you mouseovered your contacts. In the new refresh, it has been removed :)
If you still have the old version, you will probably be asked to download the new one when you sign in. The new version number is 8.0.0566
You can read more about all the latest changes and future changes to MSN Messenger on the official blog: http://spaces.msn.com/members/MessengerSays