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.