This documentation applies for JabRef 3.7 or later.
MathSciNet is a searchable online bibliographic database. It contains all of the contents of the journal Mathematical Reviews (MR) since 1940 along with an extensive author database, links to other MR entries, citations, full journal entries, and links to original articles. It contains almost 3 million items and over 1.7 million links to original articles (Wikipedia).
To fetch entries from MathSciNet, choose Search -> Web search, and the search interface will appear in the side pane. Select MathSciNet in the dropdown menu. To start a search, enter the words of your query, and press Enter or the Fetch button.
Then, the results are displayed in the import inspection window. In case an error occurs, it is shown in a popup.