Right now mdEditor is in Beta at https://my-beta.usgs.gov/mdeditor/
Right now you can enter data into the app either by hand or using a combination of a ScienceBase record and by hand data entry.
mdEditor allows users to seed (or start to populate) a record using an existing ScienceBase (SB) Record. To do this, first find your SB record in SB https://www.sciencebase.gov/catalog/
The set of letters and number after /item/ is your SB ID number in the example below the SB ID number is 505bc673e4b08c986b32bf81
Copy the SB ID number out of the URL, paste it into mdEditor, and then click the "Retrieve SB record" button:
Your record will be populated with as much data from ScienceBase as the app can pull for you: