There are a few reasons why you may not be able to create mappings for milestones and indicators in your model.
The most common reason is that you may not be assigned to the appropriate role. Only users with the permissions for Integration Architect, Insight Admin, and Model Creator users can map milestones or indicators to implementation. If you are a Business User, you will be able to define the abstract elements of milestones and indicators (such as name or type), but you will not be able to do mappings. See Integration Insight User Personas and Roles.
The second most common reason that a user may not be able to create mappings for a model is that it is not in Draft state. Only drafts of models can be edited. If you are viewing an activated version of a model, you will not be able to modify it.