I have again verified the proper functioning of Aspose.Slides for .NET 4.3.0 and have been successfully able to access the PPTX using PresentationEx class object. In my humble opinion, you may be missing the assembly reference of Aspose.Slides.Pptx in your project code. Please add the following code snippet in your application. But please make sure that Aspose.Slides.dll reference is added in your project code.
Thanks and Regards,