Step one: After installing the plugin during which it'll tell you to stop iTunes if haven't already. After it installs load up iTunes and automatically something resembling this:
should show up. If it didn't then follow these steps.
Go to Visualizer
and click on iScrobbler so that there's a check mark by it like in the picture above.
The chose turn visualizer on so that your iTunes now looks like this:
Next click the options button like in the picture below:
Next this box should now pop up:
Now put in your last.fm user info and password in the spots shown and click 'OK'
Close down your iTunes and load it back up. If it still doesn't work, you might need to restart your computer or give it some time.