There are four ways of adding references to your Mendeley Library thorugh the Desktop
Click on File and then on Add Entry Manually

Note! Be careful to select the correct reference Type. When entering author names be careful to
Open the appropriate folder in your Mendeley Desktop library and drag your PDF into it.
Mendeley will create the reference information and attach the PDF.
Check that the refrence information in the Details tab of the right hand panel is correct and edit as necessary.
Note! because Mendely is parsing the data from the PDF and using a heuristic for identifying reference information, it can sometimes make mistakes.
When you search for the Web of Science in OneSearch, the result will default to the Web of Science Core Collection. Other segments of the Web of Science database will not support the export of references in the BibTex format required by Mendeley.
Enter your terms in the Search box.
In your search results set the check boxes for references you want to collect. Then set to Other File Formats and click Export
Then select BibTex.
Note! It is a good idea to select Full Record and collect as much information as possible. Click on Export.
In the Savedrecs dialog box set the Open with radio button and click on Browse to select the Mendeley Desktop program and click OK. Mendeley will open and import your references.
In your search results set the check boxes for references you want to collect. Click on Export
In the next window, select RIS Format.
Note! It is a good idea to collect All available information.
In the Opening Scopus ris. window set the Open with radio button, click on Browse to select the Mendeley Desktop program and click OK. , Mendeley will open and import your references.
In your search results set the check boxes for the references you want to collect. Click on the ellipsis at the top right hand side of the page
In the next window, select RIS Format.
In the Export/Save dialog box, with the Output on RIS, click on Continue.
In the Opening ProQuestDocuments dialog box, go the the Open with radio button and select MendeleyDesktop. Then click OK. Mendeley will open and import your references.
Records from OneSearch need to be imported individually. After reviewing your list of search results select the ellipsis on the top right corner of the record.
Select the icon EndNote/Mendeley below the record and then click on Download. In the Opening Primo_RIS_Export.ris dialog box, set the Open with radio button and click on Browse to select the Mendeley Desktop program. Mendeley will open and import your references.
Before importing records from Scholar you must change the settings to use Refman as the Bibliography manager.
On the left top corner, click on the hamburger and select Settings
In the Bibliography Manager select Refman and save your Settings

Records from Google Scholar need to be imported individually. After reviewing your list of search results click on the Refman link of required references one at a time.

In the Opening Scholar .ris dialog box set the Open with radio button and click on Browse to select the Mendeley Desktop program then click OK. Mendeley will open and import your references.