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In my limited experience so far, I have found modules only don't show up when there is a configuration problem in a file like xoops_version.php or when the folder name is wrong (different from what is specified in the module, which I think is something specified in the xoops_version.php).
Basically, something is probably wrong with the module, but what it is, sorry, can't tell you.
Check the folder name, check that all the files are there (ie: no FTP problem), check that everything referenced in the xoops_version file is correct....
Good luck,
--Julian