Repair MBR (Windows 10) from OS X
The command fdisk can be used to copy the boot code to a destination MBR from some source file or MBR. This source is not supplied by OS X.

One way to write boot code the the MBR would be to follow the step given below.


Insert DVD or flash drive you used to install Windows 10.
Restart the computer and hold down the alt/option key.
Select the DVD or external drive icon labeled Windows.
Open a Command Prompt window by clicking on the following sequence.

Next
Repair your computer
Troubleshoot
Advanced options
Command Prompt
Enter the following command. I assume C: is your Windows 10
partition.

bootsect  /nt60  c:  /mbr