Keyboard navigation of Google Search results is available as long as Google Instant is available. As you type in your search query, Google Instant will suggest popular queries. Once you have typed in the appropriate text, hit Enter followed by Tab.
Once the results are displayed, hit Tab to go through the results. You should see a small arrow to the left of the result that is being selected. Simply hit Enter to go to the selected search result.
This answer has been tested on Mozilla and Google Chrome on Windows 7. There is a possibility that extensions that modify Chrome's default behaviour, such as Vimium, may interfere with keyboard navigation. Try disabling these extensions and use Google Instant Search again.
Further information on Google search results navigation can be found here.
When you search with Google Instant turned on, not only do you see
results as you type, but you can also navigate your search results
quickly using keyboard shortcuts. Let’s say you need a new coffee
table, so you start searching for coffee t. When the predictive text
for coffee table appears, press Tab to complete the query, then Enter,
then Tab to select the first result. Use the up arrow and down arrow
keys to jump from result to result and then press Enter to open the
webpage of the result you’ve selected.