Make sure the name is correct. Folder name and typed name in "select.def" file must match.
Remember it! Uppercase, lowercase or special characters must match.
Examine the folder you added inside the Chars folder, It has a “.DEF” file. That file must have the same name as the folder that contains it. If not, change the name folder.
If the folder name is “Athena_XI”, the DEF file name must be “Athena_XI.def”.
Did you choose “extract here” option?
Path to Character files and Character’s .DEF file must be similar to this:
The Queen Of Fighters/chars/Athena_XI/ (character files inside)
Did you extract all files from the compressed file? Is your file corrupt?
Make sure you extracted the character folder completely.
Or check the integrity of your file.
Open your file using Winrar, click on "Test" button. It will test your file and show you an alert message box.
If your file show errors, download it again. If it doesn't, the file was uploaded incomplete.
Can the character run on your Mugen version?
Mugen has 3 different version: Pre 1.0, 1.0 and 1.1 and some characters may not work with one of them.
In that case you need to test with another Mugen version.