How to find the location of the items I created in iTunes?
Select track in iTunes
Right-click -> Show/Reveal in Finder
Drag files from Finder window into new mail message


Alternatively (if dragging directly in step 3 doesn't work) you can click the Attachment icon in the new mail message, navigate to the folder opened in step 2 (or just drag the MP3 from the folder window into the Open dialog) and attach the file directly.