Hi Laura Delli Paoli,
I see you put your question in JBPM5.4 category, does it mean you use jbpm5.4 final full installer?
if you are using JBPM5.4 full installer, there is no more db/persistence.xml file, it has been replaced and unfortunately the guide for jbpm5.4 is not updated yet
http://docs.jboss.org/jbpm/v5.4/userguide/ch.installer.html#d0e609
it still shows db/persistence.xml while actually that file does not exist in jbpm5.4 final full installer.
I know you are trying to use MySQL Database, but maybe my explanation on setting SQL Server 2008 Database here https://community.jboss.org/wiki/SetUpJBPM54FinalInstallerToUseMSSQLServer2008UsingJTDS
can be useful just for you to know which files needs to be updated as of JBPM5.4
Regards,
Thomas Setiabudi