I think your XPath should be:
[Administration.Activities_Treatment = '[%CurrentObject%]']
This requires the datagrid to show entities of type 'Activities' and to be located within a dataview showing a single entity of type 'Treatment'.
If your association is really many-to-many, you can also simply use a reference set selector (instead of a datagrid) in your dataview.
I would change my relationship to a 1 to many, then you can retrieve your objects via association. Since I've been using Mendix, I've never seen anyone use a many to many relationship. A many To many means Object B is associated with multiple Objects A and thus it would retrieve information you didn't intend to retrieve.
Lets say that ParentEntity is associated with objects in ChildEntity, and that you have a 1 to many association between the two entities called ChildEntity_ParentEntity. If you have a datagrid listing all ParentEntity objects, and want to then display all of the ChildEntity objects association with a given ParentEntity, you could build a webform with a Dataview of ParentEntity, and within that Dataview, a nested DataGrid of ChildEntity which retrieves data via the association (this is the easiest way to accomplish what you want, I think). If you wanted to use an Xpath constraint, it would be [ChildEntityParentEntity - '[%CurrentObject%]']
Hope that helps.