How would I retrieve all the dismissed reminders in Outlook 2010?
It is not possible to restore already dismissed reminders.
But you can open the calendar view and search by modification date:
For example, I would type
Your question asks to "retrieve all dismissed reminders". I am not sure if, by "retrieve", you mean to view a list of all dismissed reminders or if you want to "undismiss" them. I've been looking for a solution to the latter for a long time and just now stumbled upon it. You first open the meeting or appointment (I double-click the event in my calendar). At the top, you will see this:
If you dismissed the reminder then the Reminder field will say None. Change it to a new interval and then click Save & Close. You should get the reminder again.