I have the Antix base with Gnome installed and want to install a deb file I have but don't know how. It's an older version of Audacity that I prefer to the one in Synaptic.

Hello timo.

1. Download the deb file of your choice to a folder eg /home/timo/deb

2. In a terminal cd to the folder ie cd /home/timo/debs

3. su to root

4. dpkg -i name-of-file (use the tab button for auto completion)

5. If there are errors in installation due to dependencies
apt-get -f install

6. You will have to add the app to the fluxbox menu, or use a terminal or Run in the menu

Got it to work but had my first foray into dependency Hell. Oh well, a learning experience indeed. Anyway, it works fine now. Overall I'm quite happy with M7 Antix so far. I was already big fan of Antix Spartacus (mainly because Audacity and Youtube worked right out of the box).

That is part of the problem with using older packages. Not usually the package itself, but the dependencies needed.
In these cases it is better to compile from source tarballs, but it is more 'complicated' or bite the bullet and use the one in the repos.