Site Search must be enabled to view reports on user search activity, including those searches that return zero results.

Set up Site Search

1. Sign in to your Google Analytics account.
2. Select View Settings from the menu on the right.
   
3. Under Site Search Settings, click the Site search Tracking switch to ON.
   
4. Enter query in the Query parameter field and check the Strip category parameters out of URL checkbox.
   
5. Click the Site search categories switch to ON.
   
6. Enter cat in the Category parameter field and check the Strip category parameters out of URL checkbox.
   
7. Click the Save button.
   

You may now view Site Search reports in the Reporting tab by selecting Behavior, then Site Search.