Once you’ve installed BimlExpress 2017 and open your first Biml file, you will probably notice immediately, that the screen is split horizontally – that is exactly for the preview pane.
If you need more real estate for your actual code, just click the „Hide“ button at the lower left corner.
To actually get a preview, click the „Update“ button on the lower right:
That’s a pretty nice feature. It can be hard sometimes to debug Biml issues because you’re often writing code to write code to write code.