I was wondering about that and decided to shoot an email to CAAPID, got this as a reply: You could either include it under Experiences or the Other Relevant Information Tab
Either way, you might want to elaborate on what you mean by mandatory; there are certain countries where receiving a DDS/DMD degree requires the completion of a one-year internship program first. Whereas other countries might issue a degree regardless of whether or not you did an internship. In those cases, an internship is mandatory for licensing, not for obtaining a degree.
Hope this helps!