Code for that is not much different from 16 series. In fact aside from the PIC18 specific initializing software, I think you can simply cut and paste assembly code from the PIC16 family with very little change.
Making code for the USB is a much bigger step however. I have a PICDEM FS USB evaluation kit. It came with bootloader and sample firmware. Luckily they are also available for download from Microchip's website. Download the file MCHPFSUSB.ZIP. Unzip and install. The source codes can be found in the install directory.
**broken link removed**
You may also try Jungo Windriver USB development software. They support the PIC18F2550 chip. There is a 30 day trial version on their website.
I've built USB based on PIC18F2550 or 4550. (USB GEN and USB CDC)
I've used HTPICC18 to built firmware on PIC, and Borland Delphi 7 to write software on PC.
PC software generated by Windriver is very complex, it read Endpoint directly.
You should use Microchip's DLL. You can use Visual Basic or Borland Delphi to do it. It's very simple.