How would I retrieve all the dismissed reminders in Outlook 2010?
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.
It is not possible to restore already dismissed reminders.
But you can open the calendar view and search by modification date:
In Outlook 2007 or Outlook 2010 use modified:date format in the search field.
For example, I would type
modified:2.10.2012 to see items I might have dismissed today
It's not 100% accurate as many things can modify the item, but if you do this soon after dismissing the reminders, it should be 100% accurate.
I accidentally dismissed some old reminders and the other answers didn't work for me for recovering them, so I restored a recent backup and reverted to that temporarily which brought them up. If you don't have a backup, there's a possibility you can use the "Restore previous versions" option in the right-click menu of the PST file (or its parent folder) in Windows explorer to get a recent version.
To be safe, I closed Outlook, backed up my current PST, and disabled my network connection before restoring the old PST.