Start the dialog editor. You will already see a new form.
You can save it anywhere. Then switch back to the InstallAware IDE, and while the Project Manager is showing, right-click the Dialogs node, browse to your dialog, and add it.
Then you will be able to use the Display Dialog command to display this dialog in your setup.
Note: You need the Studio edition to be able to perform the above tasks. Also, you want to switch to the MSIcode view.