1. The old plugin on free-codecs.com is outdated/deprecated.
2. The Nullsoft transcoder comes with Winamp 5.572
3. You can only transcode to mp3 (with the Transcoder) with Winamp Pro
To transcode flac to mp3 with Winamp Pro, the following items need to be checkmarked
on the Choose Components page of the installer options
Multimedia Engine -> Audio Playback:
-MP3 (obligatory, no option to uncheck) = in_mp3.dll
-FLAC Support (in_flac.dll)
Multimedia Engine -> Audio Encoders:
-MP3 Encoding (enc_lame.dll | lame_enc.dll)
-Transcoding Tool (ml_transcode.dll)
(checkmarking any media library plugin will also automatically install the media library, gen_ml.dll)
Then you'd select the FLAC files in the library or playlist, right-click the first selected and "Send To -> Format Converter"
Set up the transcoder settings to taste, etc etc etc.