Note: Your User Permission Profile needs the "Server Setup" Permission enabled in order for you to change/add these.
For Purchase Order Custom Fields you can setup different "templates" each containing a different set of fields.
To just use the Default Template, choose "Edit" in the Custom Fields Setup window. 
You can leave the "Default" Name or enter a new name .
Add the needed Custom Fields through theicon.
Read more information on the different (Widget) Types of Custom Fields.