Hey Joe,
My intention was to keep Site Membership Pro from bypassing the core security which only allows Admins or Role Admins to grant roles.
But it is a bug, in that the link should be visible to anyone who has edit permission, I only intended to lock down the editing of Membership Definitions so that only Admins and Role Admins can define which roles are attached to a Membership Definition. Once the Membership Definition(s) are created my intention is that anyone with edit permissions can manage Membership Products, notifications etc.
I will fix that and put out an update soon. In the meantime you can work around it by by using a user who is either in Admins or Role Admins.
Thanks,
Joe