Hi @medbh.campbell , welcome to the Auth0 Community!
The Enterprise Connection Buttons are only supported with the New Universal Login, as stated here on New vs Classic UL docs.
Also, for future reference if you do look to use New UL. If you have just the single Enterprise Connection you may find it will always automatically redirect to that connection, but to always show a button, you will need to ensure there are other connections (this could be just a Database connection with signups disabled) available to show the New UL prompt.
Hope this info helps!
Thanks