Grey literature are publications that are created outside of the standard publishing process. Examples of grey literature include reports, theses and dissertations, conference proceedings, patents, press releases, and clinical trials. They often are not indexed in article databases and thus can't be imported with metadata or matched within Papers. The process for importing non-indexed grey literature will be the same as manually entering a new publication.
Have a PDF?
If you have a PDF, you should first import it as a new reference by clicking on the “Add” button and choosing “Add Files”. You can then select the publication type and use the PDF preview pane to easily copy citation information from the PDF into the fields. Be sure to click on "Save" once finished editing the record.
Otherwise use Manual Entry
If you don’t have a PDF, you can manually enter the publication by first clicking on the "Add" button followed by "Add Manual Entry". Select the publication type and then enter in the appropriate citation information into the record fields.