Ok,
I download songs from Limewire, I save it,I try to upload it in Itunes and it doesn't work. It saves as an M3U in limewire.

m3u are just playlist files. Go into your LW library, click on Saved Files. Now click on Explore. This will show you where your LW downloads are saved & this is the folder to look in when adding the songs to iTunes

m3u files can be imported into iTunes "if" the original files position at the time the playlist was created has not been changed. Otherwise the m3u file won't work. See Playlists & how to create them (click on link) & earlier about m3u files. m3u playlist files can be opened by any audio program that recognises m3u on condition the original files are still in their original location.