Sorry but I do not understand what the problem is.

we have just replaced the defrost timer and thermostat a couple days ago I checked the refrigerator yesterday morning when I got up and it was cold.
It is supposed to be cold, after all it is a fridge.

I called my brother in law who installed it for me and he had me turn the knob on the defrost timer and after that it has not cooled anymore can you tell me where the knob needs to be set
It is a timer so it is either on cool/run or defrost mode.
Goes into defrost usually once every 8 hours or so for half an hour approximately.
Note: there is no knob on a defrost timer, it only has a cam that can be turned with a screw driver.
If you set it at defrost it should automatically go to cool/run mode (compressor on) in half an hour or so.
So if you want to force it into run/cool mode, turn the cam till compressor comes on, compressor fan comes on and or evaporator fan comes on.

Sorry but I am still missing something
we have just replaced the defrost timer and thermostat a couple days ago I checked the refrigerator yesterday morning when I got up and it was cold. I called my brother in law who installed it for me and he had me turn the knob on the defrost timer and after that it has not cooled anymore can you tell me where the knob needs to be set
If it was cold why would you turn the unit to defrost since it seemed to be working.

OK, turn it till the compressor comes on or the compressor fan or the freezer's evaporator fan. Normally all three of these will come on at once.
This will tell you that you are not in a defrost mode and the unit should cool down
If they are on now then your problem is not with the defrost timer.

If it does go into a cool/run mode then odds are high that your defrost timer is not advancing. With a pen put a line on the knob and the machine frame. Let it run a couple hours if the lines still match up the timer is not advancing.