You can see previously used addresses by drilling down into any transaction and viewing the transaction details (double click on the transaction in the Payments screen). The last screen in the wizard is the payment request that has the address in it.
I am recently using Multibit HD. Instead of starting a new thread it might be better to bump this one up since my query is about the same topic.
Anyway, I assume that used addresses cannot be seen in Multibit HD except by viewing the transaction details? So if I want to save my used addresses I would have to do this manually and copy it somewhere else?