Because of the way the lens on the Canon G series works, you cannot screw a lens or filter directly on.
Instead, these cameras are designed to accept lens adapters that click on.
NOTE: Be very careful when screwing adapters and lenses on. If you cross thread them they won't go on properly.
Here are 3 examples of what the G10 looks like with an adapter and lens/filter:
G10 Camera with Wide Angle Lens
Steps:
- Remove Bezel around lens
- Install adapter tube
- Carefully screw on Macro Lens

Canon G10 with Telephoto Lens
Steps:
- Install adapter tube as above
- Screw on telephoto adapter ring
- Screw on telephoto lens

Canon G10 with Filter
Steps:
- Install adapter as above
- Screw on filter
Canon Powershot cameras available here.