Go to Tools>Options>Saving to select the place where you want to save your downloaded files. If you're already happy with where these files are being saved, then note down the path. It'll be something like:

My Mac\Applications\Limewire\Shared

Now got to Tools>Options>Sharing, click on Add and select your shared folder.

If you can't do this, then there is something wrong with your installation of Limewire. In this case try reinstalling Java and Limewire (both available on the Limewire download page).