Are you trying to replicate this example?
https://slicer.readthedocs.io/en/latest/developer_guide/script_repository.html#export-a-segmentation-to-dicom-segmentation-object
According to this statement in Slicer core, the “DICOMSegmentationPlugin” comes from a Slicer extension. You would need to install Quantitative Reporting before being able to import DICOMSegmentationPlugin.
@pieper This probably brings up a point that the Script Repository needs to make mentions of any dependent extensions that are required for various examples.