I am trying to create multi-tier-combo test step, can you please any one guide me.
Not exactly sure what you are looking for. You can find "multi-tier-combo.tst" file <Devtest_home>/example/Tests folder. To run this test case, you need to have the Demoserver up and running.
Thanks & Regards
Did you get chance to go through the sample test case under Devtest example project and verify the multi-tier-combo test case?