If the preview button is greyed out, it is because you are working with a PPT file. The preview function is only available with PPTX files. Try saving your slides as PPTX and see if that works.
Thank you for your response. The file is saved as a PPTX.
OK, can you give any more details about how it is not working? How far are you getting in the preview process? Does it crash, hang, etc.?