FrostWire presents you with files you can download in two ways:
1. Using BitTorrent (with many seeds, a lot faster for big files & great for file packages)
2. Using HTTP (from YouTube or Soudcloud getting files directly from their servers)
When you use BitTorrent, the '+' signifies you can open the torrent package and select a file or two out of an entire list [Handpicked Torrents].
FrostWire filters your search results by file type, so if you are under audio or video, it already breaks down the torrent packages for you showing only single files. There are no packages there, even though the files you see come form a torrent package.
For the same search, you could filter by file type choosing torrent (the last one with FW logo), and you would have the '+' available there. If you would go one by one and look inside the packages, you would probably be able to find the same audio or video files you've seen where you were filtering by audio or video.