Steps to replicate:
# Activate MP and configure a payment gateway
# Then Activate Events+
# In Events+ Addons, enable the "Payments via MarketPress Products" Addon
# Then create a paid Event and a MP Product.
# Link that Paid Event to that MP Product.
# Now go to that Event page front-end. Click "I'm Attending"
# It will say: You haven't paid for this event
Click here to purchase your ticket
# Click "Click here to purchase your ticket"
# It takes you to the MP Cart Page but the cart is empty.
The linked MP product is not added to the Cart. But the linked MP product should be added to the cart so users can pay for their ticket.