Just home for lunch so thought I'd check on my C5060...
For me to use auto focus and lock the focus, I:
- press the shutter halfway down to achieve auto focus;
- then press the AF/MF/Macro select button while still holding the shutter halfway down - this switches automatically to manual focus at that focus position;
- then press the OK button on the back of the camera to lock the focus.
The Canon may be a different procedure, but you should be able to do it!
Hope it works. Let us know if it works or not!
Al.