The switch was at the front of the printer just to the right of the paper output tray. You will see on the paper output tray there are two hinges (one on left one on right) - before the breakage the right hinge would push the switch when closed. Think 'tis white.
Once located, I tied a piece of cotton around the switch and pulled it upwards, put the case back on with the cotton kinda poking out, and when wanting to print something I did have to pull the cotton, but have tied it to the shelf the printer is on so is now always taut.
Finding the clips was the hardest thing for me. Just keep at it and prod things, but not too hard. There are also two clips around the front of the printer The service manual really should help (the above link seems to have died - I accidentally linked the google temp url for it). Go http://www.google.com/search?client=ope ... 8&oe=utf-8
there, should be the top result (description like ile Format: PDF/Adobe Acrobat - View as HTML
CANON PIXMA iP4200 072005 XX 0.00-0. Scope. This manual has been issued by Canon Inc., to provide the service technicians of this product with the ...)
Diagrams on page 20 of the manual for clips.