This export format is not implemented for PPT yet.
Thanks for inquiring Aspose.Slides.
I like to share that the support for PPT export to HTML has not yet been implemented. Actually, we are at present working over merged API for Aspose.Slides that will have single namespace for both PPT and PPTX. The said API will be released during Q4 of 2013. With merged API rolled out the concerned PPT to HTML feature support will be automatically adressed. At present we have limited support for reading PPT using PresentationEx and exporting to HTML and you can try loading PPT using PresentationEx class and exporting that to HTML.