I think there are two answers to this question.
Entering from the main entrance, and entering from the International Gateway if you're staying at an Epcot resort.
At the main gate, there is a dedicated "Breakfast Entrance" at the turnstiles. The cast member stationed there should have a list of those who have ADRs for the breakfast and will let you inside.
At the International Gateway, there will also be a cast member there to let you in, but as there will be a rope corded off before entering the park, you should have your confirmation numbers handy to get past.
I have always recommended asking for your dining ADRs printed out upon checking into your Disney resort, or getting it from any Guest Relations in the parks or Downtown Disney if staying offsite.
I've only had to walk through Epcot before park opening to get to a tour, so I don't expect you should have any problems.
