I’ve only had one report of this in the emails yet but haven’t confirmed the reason. Note that this option will be greyed out on a system that doesn’t have Google Earth installed.
Presuming that Google Earth IS installed then my first thought is that Windows has lost the file association or SketchUp didn’t inherit it properly. I would start by uninstalling & reinstalling Google Earth so that all associations are freshly created.
Failing that, I would look into re-assigning the file association for KMZ and KML to be sure they’re pointing to Google Earth.
My experience in a fresh, new copy of Windows is that I didn’t have Google Earth installed and had the greyed out box. I downloaded and installed Google Earth and immediately had a nice blue icon. So, that does confirm that SketchUp CAN see it, the next question is why it isn’t visible immediately for some users. Historically we bumped into this type of problem because Photoshop would steal the reference to KMZ, this may be a similar problem.