In the P55's aircraft folder, you can right click on the p55a.xdp file and select "Open with notepad". I would suggest making backups of your files before editing thses files, because if you screw it up it can cause the plane to not show up, crash cfs3, etc.

Quick tutorial on the xdp files weapons layout section:
Okay we going to make a layout that has 6x rockets and 2x 500lb bombs. Here's the layout code for 6x rockets and 2x 250lb bombs:
Code:

	<Loadout Name="(6)HVAR Rockets+(2)250lb Bombs">
	   <Hardpoint ID="1" PylonType="pylon_P55_0" PayloadType="A_250lb" Quantity="1"/>
	   <Hardpoint ID="2" PylonType="pylon_P55_0" PayloadType="A_250lb" Quantity="1"/>
	   <Hardpoint ID="3" PylonType="pylon_P55_1" PayloadType="A_HVAR" Quantity="7"/>
	   <Hardpoint ID="4" PylonType="pylon_P55_2" PayloadType="A_HVAR" Quantity="7"/>
	</Loadout>
 

Copy this whole section of code, and paste it directly below. But we dont want 250lb bombs on the new layout, we want 500lb bombs. So edit the code to this:

Code:

	<Loadout Name="(6)HVAR Rockets+(2)250lb Bombs">
	   <Hardpoint ID="1" PylonType="pylon_P55_0" PayloadType="A_500lb" Quantity="1"/>
	   <Hardpoint ID="2" PylonType="pylon_P55_0" PayloadType="A_500lb" Quantity="1"/>
	   <Hardpoint ID="3" PylonType="pylon_P55_1" PayloadType="A_HVAR" Quantity="7"/>
	   <Hardpoint ID="4" PylonType="pylon_P55_2" PayloadType="A_HVAR" Quantity="7"/>
	</Loadout>
 

And, of course, we want the layout to show up as "(6)HVAR Rockets+(2)500lb Bombs", so edit the first line of the code extract to this:
Code:

	<Loadout Name="(6)HVAR Rockets+(2)500lb Bombs">
 

Save, quit all programs, and go fire up CFS3.