If a diary code has not been added to the event type in the object diary events screen you get ? marks on the export.
You can add the diary codes as follows:
Click Options, then click Manager
Click Master Types, then click Object Diary Event Types
In the left column, click on the Event Type you wish to change
In the right column, click Edit
Select the Default Diary Color to use from the drop down
Make sure you enter a value into the Diary Code field
Click Save