Once you've installed Mendeley Cite the add-in can be accessed via the 'References' tab on the Ribbon in Word.
Selecting 'Mendeley Cite' in the 'References' tab will initiate Mendeley Cite in an add-in window found in the right hand side panel, where you'll be shown the sign in page.
Once you select 'Sign in' a pop-up will appear with a sign in page. You will need to sign into your Mendeley account to access your library in Mendeley Cite. If you don't already have an account you can register for one here. Once you have signed in you will see your library in the 'References' tab in Mendeley Cite.