Why is there a watermark on my Flex charting applications saying 'Flex Charting Trial'?
There are actually two different licenses for FlexBuilder (and the plug-in): Flexbuilder Standard and FlexBuilder Professional. One is the base version, which will allow you to use most of the components, but there is a more expensive version which is part of the Flex Data Visualization component set. You can tell if something is in this license by looking for (Flex Data Visualization components only) in the documentation.
There are a number of workarounds to get rid of that TextField (which you can search for online), but the moral way is to buy the more expensive license.