When you are within the “New Activity” pop up, first make sure that you have ticked the “Visible to Customer” box.
If you have written some notes and would like your notes to be visible to the customer then also make sure that the “Mark Note Visible to Customer” checkbox is ticked.
Lastly, ensure that “Visible to Consumer” is ticked on the Activity homepage for the customer. Note: attachments added in Activities can not be seen by customers. Read how to make attachments visible to customers.