How to make a test case runs on a specific machine?

If you hit run at the top of the test case, then the "Test Case Run" dialog opens. If not already selected, you'll need to select your RFT script from the drop down. At which point the dialog will have a field for you to specify which machine to run it on. RQM will select one by default for you, but if you don't like the one it selected, there's a little pencil icon beside the machine name that you can use to specify a particular machine.

Also, if you initiate the run action on the Test Execution Record then an adapter selection dialog should pop up with a list of RFT adapters for you to select.