Your mac wants to use Keynote to open files with the ppt or pptx extension.
Tell your mac to open PPT, PPSX or PPTX files with PowerPoint instead. In your FINDER find the file you'd like to open with PowerPoint, and select it. Click on the File menu and select Get Info. Expand Open with: by clicking on the triangle to the left. Select the Open with: menu, then choose the app you'd like to use to open all documents like that one.