I want exactly 30 FPS in my Application.
What is a good way to accomplish this?
Are you using SimpleGame or StandardGame? If you're using StandardGame you can set the framerate using GameSettings.
I think he doesn't mean the refresh rate of the display, but how often the game logic is updated (athough basically everyone will be guessing with such a short post). Regardless, check out any of the classes in the .app package, it's almost impossible that what you seek is not there 
keeskit, refresh rate and game rate are actually quite similar if doing work in the update thread since they are bound to each in a fixed framerate…unless of course performance prohibits reaching said framerate.
I use basesimplegame
If it's not possible to manipulate the game logic, than how can I only change refresh rate of display.
I need to fix the FPS, because I want to use the programm on two computers with similiar speed(I know that exactly is not possible^^)
…lets take a step back here. Explain exactly why you are wanting to set an explicit framerate?
Because I think it's better to regulate the FPS.
My display supports only 60 FPS per Second. jME produce 2000 Frames per Second, so it's not necessary.
The most important thing!!!
My graphic card makes "no good" sounds with 2000 Frames and it smells. When I reduce the FPS to 40 with "Work", the graphic card gets quiet and stop "burning".
I know, my graphic card won't catch fire, but I think this is a problem, so I want so solve it.
Just turn on vertical sync which is defaulted to on in StandardGame…I think there's an example in the source of how to enable vertical sync in regular games…that will limit you to 60fps (or whatever the user's refresh rate is)