There is an easy to read manual online for the older cousin of your radio called the Pro-97. It gives you examples of how to program your EDACS system in the Programming Trunked Systems section: Easier to Read Pro-97/2055 Scanner Manual
The biggest thing to remember when programming the PSR300 is that the mode for the trunked system needs to be set to ED for EDACS (F+Trunk, press MODE) and that the trunked system frequencies stored also need to be set to ED (again, MODE button) for EDACS.
The channel order is also important and called the Logical Channel Numer, or LCN. Be sure to put the EDACS system frequency in the correct order.