It is hard to tell what you're asking. Are you asking how to select a mate or how to unsuppress a mate? If you want to select a mate, use IModelDocExtension::SelectByID2. If you want to unsupress a mate, use IFeature::SetSuppression2.
Apologies if the question wasn't clear enough. My use case is following.
I am trying to add Top plane mates to two components at a distance( the two components have Right and Front mates already ) .
Unfortunately after adding the top plane mates between the two components, the resulting mates is coming out as suppressed. So I was just trying to unsupress this mate forcefully via EditUnsupress.
In order to select this Mate by SelectByID2 methods, I am looking for the selection string on this mate, so that I can select the mate and call EditSuppress on ModelDoc2 Object for the selected component. I can't find anything on mate object that I can use to select this object
Hence the post to find out how to select the mate once you have mate2 object