Not so easy to figure out what is going wrong with the information you provided but if you define a postaction on the revision on ITEM_create_rev the dataset must be attached to the revision and not the item.
NX 11 | SW 2016 | Creo 4 | TcUA 11.4
Evaluating: AW 3.4
You may want to try setting up 'automateAndLink' extension to 'Create(Teamcenter::CreateInput*)' operation post-action on your Item.
Arguments for this extensionRule will be:
TargetObjectType: <select the dataset object>
RelationName: <select the relation name>