To add candidates to a talentpool one has multiple options.
Working from the Talentpool you can use the tab 'Search Candidates'. After selecting the desired candidates from the list a button will appear that is called 'Add to Talentpool' clicking this button will directy transfer the candidate records from the Search Candidates tab to the 'Candidates tab' of the Talentpool.

If you are working from the general Candidates menu a button 'Add to Talentpool' will become available as soon as a selection of candidates is active.

Third option is to add a candidate directly from its candidate record using the 'Add to Talentpool' button from the top of the record