I doubt it has anything to do with your Mac. Most likely the songs you're trying to download are fakes and that's why they never find any sources.
The network has been flooded with fakes from different organizations that don't want people sharing files, but there are still plenty of good ones out there. For more info on fake files, see this post:
Recognising Fake files.
When your searching for files, try to vary your search criteria. Just search for the key words and not the full title. Try just the artists name, or just part of the name. Search for the song title and artist name on the same line not in the different spaces in the search. Also try the files with only a few sources(20 0r less) and a modem speed as these are less likely to be fake.
Another thing you can try is to do a "Whats New" search(these tend to be good files) and then browse the host of the files that show in your search results. It's pot luck, but you can find some real jems that way.
You can find these and other tips in this thread:
How to find music 