Theme activation error message

I am trying to activate new blog themes (free ones) and none of them will activate. Instead, I get an error message saying Download failed. Not found. I’ve tried activating 5 or 6 different themes and they all do the same thing. Can you help me figure out what’s wrong? I’ve been browsing around for a new theme for over a week now and I’ve been trying dozens of themes before today and never had this happen once. Help!