Autodesk has released Service Pack 3 for all three flavors of Navisworks 2015. This service pack fixes an irritating problem that was created by Service Pack 2 of only being able to appended models/files one at a time.
The Autodesk Application Manager has these updates on it now, or you can download it from here
I have also attached the read-meĀ file if anyone is so inclined.
Autodesk_Navisworks_2015_Service_Pack_3_Feature_readme
The BIMsider