Simbrief doesn't save in the Captain Sim .RT format, so save the Simbrief plan as a default P3D flight plan (.PLN) in Documents\Prepar3D v4 Files, or Documents\Prepar3D v5 Files!
Now you can load it into the CDU.
After it's loaded into the CDU, save it using the RTE <save (LSK4L) and it's saved as a .RT file in P3D\Captain_Sim\767-II\routes.
Whenever you load it, it will appear as a COROUTE.