Hi meels86!
The error might be happening because a style file or the style file editor is open while you are trying to run an input file; the "executeDRT" part of the error might refer to the .drt extension used by the style files.
Try running DirectRT and make sure that you don't have the Style editor open. Please let me know if this helps you get started using your copy of DirectRT!
Cheers!
Psychboiler