By default, MySQL is enabled with strict_mode ON.
Mambo currently does not fully support MySQL5.x in strict_mode (this is on the roadmap and will be dealt with when we move to database portability so Mambo will run on other database servers, not just MySQL).

So, for now, strict_mode needs to be turned off.
This can be done in two ways...

Open your "my.ini" file within the MySQL installation directory, and look for the text "sql-mode".
Find:

# Set the SQL mode to strict
sql-mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"

Replace with:

# Set the SQL mode to strict
sql-mode="NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"

Or, you can run an SQL query within your database management tool, such as phpMyAdmin:

SET @@global.sql_mode= '';

If you have any questions about this, just post on the Mambo forums.