I'm using Debian stable (squeeze) and found this command
add-apt-repository but it's not found when I try use it.
I'm sure I've used a command before which added a repo but I can't remember what it was.
You can find the command apt-add-repository in Debian as well as Ubuntu. That command, or rather executable, is as the previous poster mentioned, in the python-software-properties package.
If you want to use the command on Debian, simply install the python-software-properties package this way;
Then you'll be able to call the command this way;
This command is contained in package python-software-properties.
Maybe you used this command in ubuntu (it's preinstalled in ubuntu karmic)?