You can't directly upload a powerpoint file on to Journo Portfolio because there isn't a good way to make them viewable on the internet, and forcing users to download every article and then open in in PowerPoint is a poor experience.
But there are still a number of ways you can add a slideshow to your portfolio:
Upload it as a PDF
If you export your PowerPoint as a PDF you can easily add it to your portfolio by clicking 'Add Article' on the dashboard and then choosing to upload a file. Most browsers support viewing PDFs in the browser making it a much better experience.
How to add PDFs to your site
Display a PDF on your site or make it a link that your viewers can download.
Use an Online Service
Some online services such as Google Drive allow you to edit, create and view powerpoint files in the browser. You can add your slide show to Google Drive's Slides and then add the URL to view it as a normal article (Add article -> Add by URL).
If you are using a link to an online slideshow, ensure you are using a publicly accessible or sharing link to add it, as otherwise people who are not logged into your Drive account won't be able to see the slide show when the click on it.
Convert it to an Article
The last option which is more work but could be a better experience for visitors to your portfolio is to publish your own Blog/Article written directly on Journo Portfolio (Add article -> Publish a Blog).
You can add any images and content from the slideshow directly into the article.