Well, the reason why this is happening is that those web dialogs are rendered by your default web browser. In the case of the Mac, that's Safari. You cannot choose what browser Sketchup uses to render the web dialogs.
Apple a while back pushed an update that caused this bug.
There are two choices to fix this:
1) You head over to SketchUCation and follow this thread. There you will find two solutions. One will show you how you can downgrade your Safari to a slightly older version that didn't have this bug. While this isn't all that hard to do, it's a pain.
The other solution looks seemingly brilliant, but requires a software engineer or a 14 year old to pull off...
2) You can upgrade to Mountain Lion. After seeing this bug, a lot of Mac Sketchup users were hesitant to upgrade, I was. I was lucky enough to have dodged the Safari upgrade on Lion so I never had to experience this bug on my production machine. I have tested Sketchup on Mountain Lion since hours after it's release and I can say that it works flawlessly in every regard. So go ahead and give Apple you $20 dollars, it's well worth getting rid of this bug if you use web dialogs every day.