I had the same error. Am really looking forward to being able to handle both event payment and tracking registration information through this. Any help on why this error comes up, though? And why there is not an "Add to Cart" link on created events? I made the recommended change to uc_cart.module.

EDIT: I also just noticed that when uc_events is enabled, my regular products show up with two "Add to Cart" links and two "Quantity" fields. Is there a way to address this?