Hi,
From the link you provided, its full of feedback from users who have built the unit, last entry Dec 2009 so think you would be better posting there.
The pics adc input can only handle 0 - 5v dc and needs a suitable network for other voltages.
Resistor R3 is 5 ohm for the backlight .
It seems to be a neat little project but be aware that most such scopes have very limited abilities, as the author acknowledges.
If you are wanting to see digital signals then you can use the Logic Analizer function of a Pickit2, if you have not got one then the 2550 you have is the heart of a diy Pickit2 Lite, just add a few cheap resistors and transistors etc, its plans are shown in the Micro forum.