The inspector is not used to define fields at all. This is done via either table view or Manage | Database | Fields. I recommend using manage | database. Once the field is defined, you can use the field picker or the field tool to add it to your layout and then you can use the inspector to format that field as a drop down list--either by selecting an existing value list or by defining a new one.
If by "dynamic" you mean a value list that displays different lists of values depending on prior input by the user, you may find "Adventures in FileMaking #1 - conditional value lists" helpful.