I am trying to add a bookmark (www.google.com) to a NOW pane with no luck. Is it possible ?

Not directly, you can only add application links - however, you could add the bookmark to the Web application (Opera) then do an application link to Web in Now!

I created a new Now pane called favorites, and under pane settings I have an option called "Content Links". Here I see a list of any previously bookmarked sites.