In Word and PowerPoint for Mac 2011, you can change the compression behaviour for a placed bitmap image via the Format Picture ribbon. Double-click the image to bring that ribbon to the front, then click the Compress button and choose the option Keep Current Resolution. Or, choose 'File' > 'Reduce Size' … This needs to be done before the presentation is saved with the newly placed image.
This is different for vector graphics placed as PDF files, which PowerPoint will nonetheless compress as soon as the presentation is saved with the newly placed PDF file. This can only be avoided in MS Office by using vector graphics in the .WMF file format, which can be exported from most of the common vector graphics editors in use, e.g., Inkscape, OpenOffice Draw, Adobe Illustrator, etc. (Not so for PDFs or WMFs exported by bitmap editors like GIMP, Photoshop, or Pixelmator – these are bitmap images only.)