The directory private does not exist

I have Drupal 5 installed and recently I tried to install Acidfree and File Manager Module but I receive error messages in both of them.
For instance I would like to solve this file manager problem. Is it a permission problem?

I get this message

The directory private does not exist

and this

A file system path where private access controlled files will be stored. This directory has to exist and be writable by Drupal. This directory should not be accessible over the web. Changing this location after the site has been in use will cause problems so only change this setting on an existing site if you know what you are doing.

I don't know what I'm doing. So how can I solve this problem?

Thanks
Gabriela