J'ai trouvé ce site la ...
http://mathieu.agopian.free.fr/prog_pic/install_c18/
que j'ai suivi à la lettre

Malheureusement quand je compile j'ai encore des erreurs que je ne comprends pas

... je vous les donne :
Clean: Deleting intermediary and output files.
Clean: Done.
Executing: "c:\mcc18\bin\mcc18.exe" -p=18F4550 /i"C:\mcc18\h" "main.c" -fo="main.o" -scs
MPLAB C18 v3.20 (demo)
Copyright 1999-2005 Microchip Technology Inc.
Days remaining until demo becomes feature limited: 60
WARNING: The procedural abstraction optimization will not be supported when the demo becomes feature limited.
Executing: "c:\mcc18\bin\mcc18.exe" -p=18F4550 /i"C:\mcc18\h" "usbmmap.c" -fo="usbmmap.o" -scs
C:\MCHPFSUSB\fw\Boot\system\us b\usbmmap.c:152:Error [1027] unable to locate 'system\typedefs.h'
C:\MCHPFSUSB\fw\Boot\system\us b\usbmmap.c:153:Error [1027] unable to locate 'system\usb\usb.h'
Thread would not exit normally.
Could not read language tool output from pipe. (5) <==== je parles de cette erreur
BUILD FAILED: Tue Apr 29 11:07:51 2008
Seriez vous ce qui se passe ?