FLAC is lossless so to preserve the audio quality yu should convert directly to Apple losses format. If you don't care about audio quality you can convert to MP3 AAC or whatever
1) Install
2) Open the FLAC Frontend
3) Click "Add Files" and add your flac file
4) Click decode. It will dump the wav file that the flac file contains.
5) Import the wav file into iTunes. Depending upon your "Import Settings" in iTunes, the wav file will then be converted to Apple Lossless (lossless) or mp3 (lossy).