Just like with the Google repository, the first thing to do is import the key file. Open a terminal and enter the following:
wget -q http://apt.last.fm/last.fm.repo.gpg -O- | sudo apt-key add -
Now, open up your /etc/apt/sources.list again:
sudo nano /etc/apt/sources.list
Now you need to decide which version of the repository you want to go for, stable or testing. If you go for stable, enter the following on a new line:
deb http://apt.last.fm/ debian stable
If you're a bit more adventurous, and decide to go for testing:
deb http://apt.last.fm/ debian testing
Press enter afterwards to move to the next line, then save and exit. Now, update apt-get:
sudo apt-get update
Now, you can install the Last.FM player with the following:
sudo apt-get install lastfm
Note that as it streams in MP3 format, you'll need to have already installed the appropriate codecs in order to enjoy it.
If you haven't already joined, you'll need to sign up to use the service. But don't worry, it's free to use (although you get more features if you subscribe). You'll need to start listening to it to build up a profile. This is now easier as they now offer many full tracks to listen to online, and will hopefully soon be integrating this into the player.
Have fun with Last.fm, and feel free to drop by my profile to say hello!