Hi Franck,
Under normal conditions, AtoM redirects you to the installer when the application failed for whatever reason *and* the configuration file config/config.php does not exist. It happens here:
sfInstallPluginConfiguration.class.php#L12-L22.
If you want to load a new database into a fresh installation of AtoM the right thing to do is: (1) install AtoM first in an empty database, (2) one you are done, drop the database and create it again using the mysql cli, (3) load the database with "mysql ... < database.sql", (4) rebuild search index... and done!
Step #1 will create all the necessary files in AtoM, e.g. config/config.php, apps/qubit/config/settings.yml, etc... and remember to set the right filesystem permissions before you start the installation otherwise the application won't be able to create these files!
I hope that helps! Don't hesitate to ask more questions.