The optimal is compiling stockfish yourself, as the precompiled versions are slightly limited (they don't use SSE instructions/prefetching).

Nevertheless, you can download the precompiled engines from the Stockfish site, and extract the one which best matches your CPU/OS architecture to a directory in your %HOME. The download is a zip file, and the precompiled engines are inside the /Linux directory.

Make it executable ( "chmod +x stock*" in a terminal window) and then point the engine on the scid preferences. You are ready to use it.

The above small tutorial explains well the permissions issue in *nix systems.

If you want the engine to be used by many users, extract it at a system-wide directory, e.g. /opt (you will need superuser status to do that) and then use

sudo chmod 0755 stock*

to make it accessible and executable by anybody.

You could also chown the file to be owned by root, but AFAIK Ubuntu has no root user by default.