There are two ways that you can add a new reference manually to your EndNote library:
1. Click on the green plus button on the EndNote toolbar 
2. Go to References (menu bar at the top of the screen), then select New Reference.
Either of these options will take you to the window where you can enter the details for your reference manually. Make sure you select the correct reference type (book, journal article, website, etc.) for the item you adding from the drop down menu.