Then you must be using a really old mIRC version, where these commands don't exist yet.
The file handling commands were introduced in mIRC v6.1.
Any version beneath that won't work.
You can find out your version by typing this in mIRC: //echo -a $version
You'll need to update your client in order for the script to work, or I could modify it to use another approach instead of the file handling commands.