In the cluster case the entire cluster must share the same database so you can't use the default embedded one. You'll first need to create the timer table in the database of your choosing. You can find the appropriate .sql scripts for each database in $GLASSFISH_HOME/lib/install/databases.
Then create a connection pool that points to your timer database. Then create a jdbc resource that points to your connection pool. Finally, set the Timer Datasource attribute via the admin console to point to your jbdc resource. There's no need to set mappedName on @Resource itself.
http://docs.sun.com/source/819-0215/containers.html#wp1023179
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