If you want to edit file in muCommander with F4 on mac, you should
1. create link to TextMate
ln -s /Applications/TextMate.app/Contents/Resources/mate ~/bin/mate
2. go to
/Users/{user_name}/Library/Preferences/muCommander/commands.xml
3. and edit or create file
<?xml version="1.0" encoding="UTF-8"?>
<commands>
<command alias="edit" type="system" value="mate $f"/>
</commands>
There ara some commands that can be useful fo you — http://trac.mucommander.com/wiki/CommandsXml