PubMed is a search engine indexing citations from biomedical literature collated by the United States' National Library of Medicine. Some items are available open access from the integrated PubMed Central collection.
Search PubMed as usual.
From the results list,
Select the desired references
Select 'Send to' option near the top of the page
Choose 'Citation manager' from the dropdown.
Click 'Create file'
In some browsers your records will be downloaded in a file displayed at the bottom of your screen. If so, leave alone for now. If asked to 'Open' or 'Save' the file, choose 'Save'.
Go to EndNote Online,
Select the 'Collect' tab.
Select 'Import References'.
A popup window will display.
'File': Choose the relevant .nbib file (from your downloads folder).
'Import Filter': Select the 'PubMed' filter from the dropdown menu.