How do I find where the file for a particular mp3 that I can see in iTunes is?

Select the song in iTunes and type ⇧+⌘+R to trigger Show in Finder.

Alternatively, you can also right-click the item in iTunes and choose this option from the contextual menu.

From there, you can navigate up the folder hierarchy by holding the ⌘-key and clicking on the folder in the window title bar. You might also want to enable Show Path bar in finder.

There are three ways:

• Get Info for the track or use ⌘+I. Navigate to the Summary tab, and it says the file's path in the Finder.

• Right-click the track and select "Show in Finder," and the file will be selected in a new Finder window.

• Open iTunes preferences. Go to the General tab and make sure the "organize files automatically" option is ticked (that isn't the real name of it; I'm not at a computer with iTunes). The iTunes file hierarchy in Finder will be organized according to Artist and Album, making it easy to navigate manually.