If you are having troubles finding location based reminders on your iOS 5 device you are probably using an Exchange based email account. If you have an Exchange account setup on your iPhone or iPad the new reminders app in iOS 5 will be using your Exchange Tasks List. It seems that the Exchange Tasks don’t support the metadata required to have a location based reminder.
To fix this issue you need to add a new Reminder List to your device. This can be easily done by tapping the Edit List button on the main reminders screen (this button has three horizontal lines on it).
Click Edit in the top right corner.
Under the On My iPhone section in the list, tap on Create New List…
Give it a name.
Click Done, and then Done again.
If you swipe sideways you will notice you have a new Reminder List called whatever you named it above. If you now add a reminder in your new list, then go into that reminder and tap “Show More…” you will now have to option to tap on Remind Me, At a Location.