Install Win10 x64 on MacBook Pro mid 2011 with Boot Camp
The correct way to burn the Windows iso file to a DVD using El Capitan would be to do the following.


Find the iso file in the Finder.
Right click on the file and select Burn Disk Image "File name" to
Disc....
Follow the instructions given in the pop up window.


If the Boot Camp Assistant application will not recognize the external DVD drive, then you can try the following.


Insert the burned DVD in the external DVD drive.
Restart type Mac and hold down the option key.
See if the DVD icon appears with the label "Windows" in the Startup
Manager.
Select the icon and see if you can boot from the DVD.


If you can boot from the DVD, you probably can use the external DVD drive to install Windows. First, you will need to create a MSDOS (FAT) formatted partition to install Window on to. This usually is the 4th partition on the drive.

If you can not boot from the external DVD drive, the one possible option would be to install directly from the Windows iso file. See web sites here and here for some examples of how to do this.

By the way, how did you acquire the Boot Camp Support Software? Did you use the Boot Camp Assistant to download or did you download from a web site?