Hi,
my.cnf might not be installed by default. MySQL just uses default configuration values unless you create or copy one of predefined configs to /etc/my.cnf.
Slow query log is disabled by default and if you need to enable it add the following in the /etc/my.cnf and then restart MySQL:
--log-slow-queries[=file_name]
If no file_name value is given, the default is the name of the host machine with a suffix of -slow.log. If a filename is given, but not as an absolute pathname, the server writes the file in the data directory.
I hope this helps.