To add a reference, place your cursor where you wish to insert your citation then on the EndNote tab select Go to Endnote.
This takes you to your EndNote library. Navigate to and highlight your desired reference and click on the Insert Citation icon.
From the EndNote tab in Word, you can also use Insert Citation > Find Citation. This opens up a dialogue box where you can search for a reference by author, title or keyword. Highlight the desired reference and click on Insert.
Below is a video giving an overview of inserting and managing citations with Cite While You Write (CWYW), which you may also find useful.