Go to Tools -> Options -> Shared & press Add button & browse to where the link suggested: C:\Documents & Settings\username\My Documents\FrostWire\Saved or Shared

Other than that, use your computer search function & type in extension of one of the files that's missing. Example: .mp3 & it should list all music files on computer. Scroll through each one until you find the location where your missing files are. Then add that folder to share.