Is your machine still under the Roland Warranty? The service mode is more than southpaw mentioned... but close.
If you unit is still under warranty, call your tech and have them get into service mode and make the proper adjustments if necessary. If it is out of Roland warranty, contact your tech and ask them to assist you in getting into service mode...
If I were called, as a Roland Service Tech, with this question I would be asking why do you need to be in service mode. If you are having a problem, the service tech should be your first call.
When in service mode, a service manual is an absolute necessity as well... One wrong change in service mode, such as a system switch or head rank can possibly damage the printer.