I had the same issue (not being able to 'Add an Activity or Resource' and found a simple solution that worked for me:
-go to the Course page
-select the Settings 'gear'
-select 'edit settings' (1st option)
-scroll to 'Course Format'
-change from 'Single Activity Format' (the default setting, I believe) to 'Topics Format'
-scroll to bottom and select 'Save and Return'
--> voila: I can now 'Add an Activity or Resource'!