To see how the controls in your dialog work, click Test Mode from the Options menu. Test Mode creates a functioning dialog box for testing. The list box becomes operational with a scrollbar and lines of sample text.
The Dialog Editor inserts lines of sample text. You cannot insert lines of your own text directly into the Dialog Editor. You can scroll the sample text, select it, and see how many lines and characters the box can display, and you can test the changes you make by clicking Order/Group.
You can also use Test Mode to investigate the various style options you can apply to controls and to the dialog box itself (see Dialog and Control Styles).
To quit test mode, click Test Mode from the Options menu, or press ALT+F4.