This only applies when importing a song into iTunes without having SCD open iTunes!
As from the 220.127.116.11 update, SCD might show you a similar popup:
This means, that you have the enabled SCD to automatically import the song into iTunes and also checked the option to import it without opening iTunes. A bit like this:
As the title of the alert says, SCD could not automatically find your iTunes library. For SCD to continue to import your downloaded music, you will need to find the new library yourself. It could be, that you moved it over to a shared server or anywhere else on your mac. If you don't know where your iTunes library is located at, open iTunes preferences and navigate to the "Advanced" tab. Right at the top of the window you should see where your iTunes Media library is located at. In this example, it's set to
/Users/SCD/Music/New iTunes Library/iTunes Media. Now back to SCD, click on the "Locate" button to start searching. You will only need to navigate to your iTunes library, not all the way to the iTunes Media folder!. You should have something similar to this:
Now simply click the "open" button on the bottom right to finish. SCD shouldn't ask you again where your iTunes library is located at, unless you move it or have it on a server which is currently disconnected.