A small change has been made to the method of transferring models between Autodesk Revit 2015 and Autodesk Robot 2015 which is a little different from the 2014 products.
This change will only affect users that need to transfer data via a .rtd file, if you have Revit and Robot installed on the same machine then the transfer method is the same.
In the dialog box below you can see that the .rtd file has been removed and you now need to use the intermediate file (.smxx). This has been implemented to improve the data that can be shared between applications.
When importing the .smxx into Autodesk Robot Structural Analysis you will need to use the following method shown below. You cannot directly open the .smxx file.