I just wrote a comprehensive answer, but it somehow got deleted when I hit save.
But basically, the list of people you can connect the census page to is limited by factors like the transcribed gender and the birth year of the people in your tree, and sometimes the person you want to connect seems to just inexplicably not be on the list of options.
The way to get around these restrictions is to go to the page in you family database that corresponds to the person you want to connect to, and add the census page from there by pressing the red "New Event" button and choosing "census" as the event type. From there, put in the place and time of the census and you will then get to choose the census page to connect that person to. You can even open the "Entered Name" dropdown menu to choose the correct name from the transcription, even if the site wouldn't let you connect that transcription to the right person on your tree.
Hope this helped.