there isn't a direct way....you can usually export PPTx as PDF, and I don't know why you'd insert a flattened PDF into photoshop and save it again but that would be the easiest way. Now, inseatd of asking how to get PPTx into PSD format, maybe you could explain WHY you want it as PSD, or what you need it as PSD *for* so we can better help you.
I need to enlarge, edit and then print the presentation. if not psd, is there a way to convert pptx to jpeg/jpg without compromising the quality?
try exporting the PPTx as a PDF, and then opening the PDF one page at a time in Photoshop - that's the best way... BUT, you can't enlarge it beyond what it is currently, you'll just scale it up but the quality can't be any greater than it is right now. @Maya_16: did you even read the thread? And aftereffects doesn't convert to JPG, Save As does, and Save for Web does it even better!