To install the DSON Inporter with the Install Manager click the gear icon at top-right of the DIM interface to get to preferences, then go to the Applications tab. If there's already an entry for your version of Poser check that the path is correct - click the ... button next to it and set the correct path if not. If your version of Poser isn't listed at all, click the + button at the bottom of the dialogue, set the application to Poser, set the version number and bit-depth, and click the ... button to set the path.