This post is full of inaccurate information:
Prior to iOS 9.0.2
this is the operating system for mobile devices, it has no relevance for the Mac
whenever I opened a Powerpoint file in my browswer (e.g. chrome)
Powerpoint files do not open in Chrome, chrome is a web browser
PowerPoint files will only Open in Keynote, Powerpoint, Libra Office or Open Office
I was given the choice to open it in Keynote.
PowerPoint files are opened in two ways:
1 - right clicking the file and selecting, Open With > Keynote
2 -open Keynote then go to: File > Open then select the PowerPoint file, then click Open
Before anyone can help, you must clarify the following:
Mac OS version (Apple Menu > About this Mac)
Keynote version you are using (Keynote > About Keynote)
what PowerPoint filetype you have, it will be either either ppt or pptx