I am not sure, but this works in my own WDL, so it might also work in WDL-OL:
Code:
odialog1 = new IFileSelectorControl(this, &IRECT(0, 0, 100, 100), kOpen1, &bitmap, IFileSelectorControl::kFileOpen, "", "wav");
pGraphics->AttachControl(odialog1);
I.e. replace IGraphics::kFileOpen with IFileSelectorControl::kFileOpen.