Add a sheet material word to the component definition name. i.e. MDF, sheet, plywood, etc. Or add a new sheet material word that your components already use such as 'panel' in the first page of the CutList UI. Remember to save the settings. Or you can add a material to the components which has a sheet material word in its name. You can modify a simple color's name if you don't want to have actual materials applied.
Any of those options will get the parts to show up in the Sheet Materials section instead of the solid wood components section.