Why not just ring the company and ask them?
They could be running several different groups at different venues on the same day for all you know. No-one at the company will think any less of you if you call and clarify the details with them. It's only an assessment day after all. They'll have a lot of applicants, many of whom won't make it past the first few hours, so they're not going to remember you as "That girl who lost the email".