if you want to run your jbpm with jboss 5.1.0 and mysql, then only things you need to change are :
1.the database parameter value to "mysql"
2.Value of jboss.home is to be changed to the path of the server.
Now the ant scripts to install jboss,install hsqldb server( and other hsqldb related scripts) should be ignored.
If you also want to install the examples in your database, then the database parameter value should be changed to "mysql" in both the build files. One present in jBPM_HOME/install, and other in jBPM_HOME/examples.
regards.