Juniper AC power supplies are what is called universal supplies. They work if they are connected to either 110 or 220 feeds.
The question is for the electrical panel input feed to the power supply. Is the eletrical feed sending 110 or 220 volts to the power supply?
If you are connected to 220 volt feeds then you install in pem 0 and pem 2 for full redundancy.
If you are connected to 110 volt feeds you need 4 power supplies for full redundancy and use all 4 slots. If you only have two 110 feeds you need to use both pem 0 pem1 for full power to both power zones of the mx240.