Where is the configuration file that indicates the paths of the Units ?? I could not find it
I believe you are looking for fpc.cfg which is usually located at /etc/fpc.cfg . However, when you've installed Lazarus, you should have seen the start-up window, which preforms automatic checks of the folders and detects any errors. Also make sure you've installed all three debs: Lazarus, FPC and FPC-src.
P.S. You might also want to try the latest Lazarus version, 1.8RC2 at the moment (don't forget to purge and remove the previous one, including, delete ~/.lazarus folder to avoid strange folder behavior which might actually happen after upgrading).