If the search results page has an error message at the top saying the stop you searched for is not a valid stop, try searching again and checking you have spelled the stop name correctly.
When you start typing the name of a stop or city in the 'From' or 'To' fields, a drop-down list will appear showing all stops matching what you have typed. If you have typed the name of a city, all stops within that city will be displayed. Select the stop from the list.