There are 3 ways to import references:
The file with references can be a BibTeX, RIS, Zotero RDG, etc. file or a Mendeley Reference Manager online import
:
Click Next> and select the file - Open
Select the Options:
Click Next>
The references will be imported into your library. Click Finish
2. Via File - Import from Clipboard
The copied text should be in RIS-format, like this example:
TI: When I write my masterpiece: Thoughts on what makes a paper interesting.
PY: 2006
AU: Barley,Stephen R
SO: Academy of Management Journal. Vol 49(1) Feb 2006, 16-20.
Check if all fields are imported and if the reference-type is correct.
3. Via Add Items by Identifier
At the top of the center pane you see 4 icons, the second one is the identifier. If you click on it

You can now copy/paste an ISBN, DOIs, PMIDs, etc in the field and the item will be found and added to your library