Very convenient. Rated 5 out of 5 stars

At first, only a tiny square appears on the right of "Search for ... with". So I thought it didn't work, and gave only one star to this add-on.
But after proper configuration, it works perfectly!
Like another add-on "add to search bar", you should right click search box of google, select "add a key word to search", and add a bookmark anywhere as you wish. Then you'll find that tiny square grows to a normal sub menu with a google search option. After that, if you move around or rename the dummy bookmark in your bookmark repository, the sub menu still works. It works with most search fields I have met.
I'm sorry for my ignorance. Thank you for your great work!

This review is for a previous version of the add-on (0.4). 

Configuration

It appears that You do not have any keyword searches defined - typically You do it by right clicking on any search field and choosing "Add a Keyword for this Search...". After You create keyword it will appear in "Search with" menu.