I just update to Mathematica 10 and i have installed extras.pkg but i don't know how it works & i couldn't find help.
You don't have to activate these features. They just kinda work in the background. One of them allows spotlight to look inside Mathematica files. This makes spotlight work better. The other allows your browser to display notebooks. So if a website has a notebook and you click on it as a link, it'll open up. This is just like how many browsers allow you to read PDFs in the browser instead of downloading them and opening them with a separate PDF reader.