I was told by the salesforce customer service team that it was possible to create many-to-many relationships between activities and custom objects.

I have attempted to implement this relationship but I am unable to select activites as a related object for the junction object to relate to, in order to form one of the master-detail relationships that would make up the many-to-many relationship.

Is there something I am missing here that would enable me to create the master-detail relationship?