You must be in edit mode to see this section.
This option does not appear if you've already made it into a SQL query.
The top tab must have Data Source selected. This option does not appear under Views, Report Parameters, Drilldowns, or Advanced.
In the top, right-hand corner you should see the Symantec logo and a search field; below this, a greyed out Edit button (since you're already editing), and below this the blue text 'Convert this query to SQL query'