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: 

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.

Near the end of this video there is an overview of inserting and managing citations with Cite While You Write (CWYW), which you may also find useful:

01:12:15 Cite While You Write - Insert Citation 01:16:55 Cite While You Write - Insert Selected Citation(s) 01:19:00 Cite While You Write - Configure Bibliography 01:21:25 Cite While You Write - Edit & Manage Citation(s) 01:23:58 Cite While You Write - removing an in-text citation