Connecting a PC PS/2 keyboard to Mac
This are the steps I've follow to use my keyboard, I've tried so many things that I don't know which step is mandatory: 


Shut down Mac and wireless keyboard
Plug in the keyboard with the PS/2 USB adapter
Switch on Mac


If Mac is stuck to the first gray window try to use a different USB in my case if I use the first USB the keyboard doesn't work but if I use the fourth it  works.

If the keyboard still doesn't work I've try to install PS/2 driver with the Kext Utility.

Maybe first you should remove:

/Library/Preferences/com.apple.keyboardtype.plist

At the login the keyboard should be operative so you can write the password with your PS/2 keyboard. I simply ignore the Identify keyboard window that appear after login.