Even if you are only scanning a Workgroup, you need to run the Network Data Collector on 1 pc to produce an NDF file. CDF (local Computer Data files), SDF (Security Data files) all get merged in to the NDF when they are imported.
If for some reason you don't have an NDF and cannot easily perform a Network scan, you can download a blank one below:
Simply import the blank along with your CDF or SDF files and you should be able to run the desired reports. Please note that information that is normally collected during the Network Scan will be missing.