PSIM simulation of buck chopper
You are using an N-channel device as your pass transistor when you should be using a P-channel device in this type of circuit configuration. Look at the parasitic diode component across MOS1 - you can see that it will be conducting all the time.
Change it to a P-channel device like this: -

As for not finding an isolation component in PSIM, you should be able to use a VCVS. If you do this you will be able to use an N-channel MOSFET for MOS1 but with drain connected to the input supply.