Import can be done in several ways:
When you export from firefox you will get this box
Chrome:
If you use the Chrome browser, there are different ways to export to Mendeley. When exporting, the file shows up in the lower left corner.
Make sure your Mendeley is open. Drag the file to the Mendeley icon and Mendeley will open
Drop the notepad file into mendeley and the references will show up
Microsoft Explorer / EDGE:
Edge:
Explorer:
Choose Save as and the file will be saved as a notepad file
Choose Open Folder and drag the file into Mendeley
Safari - Mac:
Open your Mendeley Desktop
Go to your downloads and choose the file you just downloaded
Drag the file into Mendeley
You can also just drop the file over the Mendeley icon and Mendeley will opn with teh references
File names:
Web of Science: Savedrecs.bib
CAB Abstract: ris.ris
Goggle Scholar: citation.ris
REX: Primo_RIS_Export.ris
Bibliotek.dk: bibdk.[dagsdato]_162952.ris
PubMed: pubmed_result.txt
There are three different ways to download web installer - whichever browser you use
In Explorer and Safari, drag the installer into the bookmark line 
(This does not work so well in the Explorer because Explorer is being phased out by Microsoft. It looks, however, like it does not work in their new browser Edge right now). So if you use Explorer, it is best to choose a different browser
In Firefox and Chrome it is a Plug-in 
and is placed in the upper right corner of the browser
You can go from Mendeley Desktop - Tools - Install Web Importer or you can go through this link https://www.mendeley.com/import/
Remember to always check the references you import from different databases and other places. Mendeley is only a program to help you with your references and errors may occur when importing
If the reference list is not correct, it may well mean your grade goes down. Talk always with your supervisor about what he / she want