So, you're a cool person.. you're hanging out on the IRC channel, #libre.fm on freenode, checking things in and adding to the buzz of the project.
But maybe you're still not using the earliest of alpha versions of the site.
Here's what you need to do...
To get an account, please go to http://libre.fm/ and register.
If your client software is configurable (see Client Support) then for this step, just configure it to handshake with turtle.libre.fm, and modifying hosts or lmhosts will be unnecessary.
Modify your hosts file (/etc/hosts) and add the following lines:
(Where xxx.xxx.xxx.xxx is the IP address listed on https://turtle.libre.fm)
On windows systems the host file works in the same way but it is located at: c:\windows\system32\drivers\etc\hosts (You have to open it with notepad to edit it)
Instructions for altering your hosts file under mac os x 10.5 can be found here.
More info about the hosts file and it's location on other operating systems: wikipedia:hosts file
Double check you have the hosts file set up correctly
Check the ip:
In windows go to start » run.
Type in: CMD
The Black dosbox will appear.
Type in: ping post.audioscrobbler.com
If you get a response, it should be working.
Change your player (Rhythmbox, Songbird, etc) to use your libre.fm username and password, and start scrobbling!
Listen to some music -- make sure you restart your player -- and you should see it appear on http://alpha.libre.fm/user/yourusername