Alternatively you can go to Google+, right-click the search bar and select "Add As Search Engine..." You can then change the keyword to "+" or "plus", and do the same thing.
That won't live update with results as you type though. Search+ pulls in recent results from Plus and lists them in the drop down. If you select a result it will open that post directly instead of going to Google+ and showing results.
Doesn't work for me, and my Chrome is up to date.
If you enter "+" into your URL bar and hit tab does it say "Search+ for Google Chrome"?