i have two computers both running OMNet++ separately,and connected via Wi-Fi, can i send message from one computer to another using omnet++ over wifi?
Yes, you can, but I'm not sure you should. Check how to set up MPI bases parallel simulation scenario in the manual, however I'm almost sure that this is not what you want to do. If you want to simulate two applications while they are speaking to each other via a physical, real world WIFI card, then you should check how to set up external interfaces in INET.