To enter a references to your library manually:
From the top menu bar, select References > New Reference
Select the appropriate Reference Type from the drop down menu. (Note that the reference type you select determines the fields that will display in the citation.)
To close the record, select File and then Close Reference. The new reference will save automatically.
The new record will be in the All References group by default. You can then move it into any group and it can be placed into multiple groups.