The current version of DNRGarmin (5.4.1) is not entirely compatible with ArcGIS 10. That is, the toolbar will show up in ArcMap 10 and it will launch the DNRGarmin application but DNRGarmin does not recognize that ArcMap is open. We have not yet begun programming the new version.
The work around for the time being is:
File->Save To->File and File->Load From->File. You can save to a shapfile and add it manually to ArcGIS 10, as well as load from shapefiles created/modified in ArcGIS 10.