This may be obvious, but the imported playlists appear under the "Playlists > Imported Playlists" tree branch, not directly under "Playlists".
As already said, in any case, the filenames inside the playlist must be valid. Also the file path must be valid if the path is included in the filename (the directory path in the filename is necessary only if the audio files are not in the same folder with the playlist).
In other words, MC does not import playlist entries that do not point to existing disk files. You can use a text editor for checking and fixing m3u playlists. Notepad would work fine.