I just learned how to do this, but it is actually pretty easy. I have an imac too. In your Limwire folder, there are a few other folders, such as shared, incomplete, etc. In my computer, this limewire folder is in my applications. So, you also want to open up your itunes folder, which i think in my computer is in documents. Anyways - you want to highlight any song, or all th esongs you have downloaded from limwire, each song has its own icon in the limwire shared folder, and drag it/them over to the itunes icon. It will copy it and you will be able to use it. As far as I know.

Good luck