Conduct your search in the usual way, and mark the records you wish to add to Mendeley.
Click the Export icon; this will open a new window.
On the new window, select RIS from Format drop-down menu, select Citation, Abstract from Fields drop-down menu if you wish to include abstract in the references, and click Export to save the RIS file.
Open Mendeley Reference Manager, click Add new - Import Library - RIS (*.ris), navigate to the RIS file you saved, and open it. The references will be added to your Mendeley library.