Formidable comes with two standard plugins you may use to invoke and run your XML application. All you have to do is to provide the path of your XML application.
It has to be of type "Insert plugin", of course.
In the Plugin list, select "FORMIDABLE_INT cObj (not cached)".
It may be located anywhere in your TYPO3 filetree. Provide the path as relative to the TYPO3 file root.
You may now display the page in your website to see if everything is OK. If not, Formidable will inform you about the problem with a Mayday screen.