I have been using the app for nearly a year - when i click register a classmate - then i search for the classmate - once i click on their name - it does not take me to purchase tickets - it takes me to a screen which is the event details. Help!

The most likely answer to this is that the registration deadline for your activities has passed. In order for you to make this registration, the activities need to be available to be registered for and therefore you need to push out the deadline to a date later than today... and publish.