Hi. I hope I have read the FAQ throughout before posting this.
Could you explain why when I try to scan a directory where MP3 have been encoded with Nero suite, the directory appears empty, whereas if the same files have been encoded with any other software, they will appear normally ? Is it a question of the mp3 dll/codec used by Nero ? Is there a way around it without having to recode, in my case, hundreds of files ? Thanks in advance for your help.

Make sure you have filter disabled. Toggle the 'F3' button first

But apart from that not sure whether this is an issue with mp3tag as so much your encoder app.

Have you tried using a different converter. Do a quick goggle search. There's loads out there that use the 'lame_enc.dll' and to be honest I have stopped using Nero products a long time ago. Encoding video esp!

I personally use dBpoweramp Music Converter which support multiple core's and has a right click shell extension for converting your files...

Two things:
if you use mp3tag's directory selector in W7 then this will show only the directory but no files in the directory. This is a W7 feature.
If you are sure that the files should be loaded then check the file suffix and check whether this suffix is included in the list of suffixes in Tools>Options>Tags.
If the file suffix is missing, include it and retry.

(I am not sure about Nero's mp3pro format ... which unforturnately is the default for mp3 files)

Thanks a lot. F3 did the trick.