Go to the Listings tab in CORE and then click on the New button.
This will bring you to the manual entry page. Fill out the highlighted areas. ( Make sure to add a Seller where it says Seller: click here. If you do not you will not be able to create a listing.)
After you have the information filled out, click on the Create Listing button. This will bring you back to your listings and you can fill out the rest of the information for your manual entry listing (beds, baths, status, year built).