I need to know how to introduce a new equation for fitting data. I have already the fdf file which is working in Origin 7. I have tried to add it but it is not working in origin 8
You can drag-and-drop the FDF file to Origin workspace to add it to desired fitting function category. And then use it to do fitting.
Alternatively, you can open Fitting Function Organizer tool(Opened by pressing F9), click Add button to browse this FDF file to add it to selected category and then edit the function for your own need in this dialog.