The latest version of Word for a 365 subscription is Word 2016. To get the EndNote X8 tools installed in Word 2016, please see this article: http://endnote.com/kb/141156.
Keep in mind that the EndNote X8 tools will appear as a tab in the Ribbon as indicated in that article.
While this video was created for EndNote X7 and the icons are different in X8, it may help you get familiar with the EndNote tab in the Word 2016 Ribbon: