1. turn off and disconnect the Canon PIXMA printer, both from the power source and from the computer. Leave it that way until asked to by the installation routine, or until the installation is done.
2. Remove the printer from the printers list, if it is listed there.
3. Remove any program associated with the operation of the Canon PIXMA MG3550, if there are any.
4. Reboot your computer.
5. Download your new driver for this printer, for MAC OS 10.7 of course, from:http://www.canon-europe.com/Support/Consumer_Products/products/Fax__Multifunctionals/InkJet/PIXMA_MG_series/PIXMA_MG3550.aspx?DLtcmuri=tcm:13-1081558&page=1&type=download
- scroll down to the bottom of that page, click on the Accept & Download button.
Then download the MG3550 Full Driver And Software which is a DMG file.
6. Install software from the DMG file... which should, once downloaded, be mountable as a disk-like entity, then readable from for various files/programs, to run or copy as needed for the install.
That should do it. If there's a problem at any stage, stop short, note it down, and Reply to me about it, please.