Ok, I went through the steps and didn't have any problems, so I would say this is not a problem with 10.3, but some other issue. There are some troubleshooting tips in the checklist PDF:
-The DDP Script Tool in MapSAR won’t create Assignment maps.
The error message is:
Assignments Selected are [1, 2, 3]
There is no assignment # 1 in the system
There is no assignment # 2 in the system
There is no assignment # 3 in the system
a. I’ve found this to happen when you choose the wrong SAR Incident folder containing the Assignment DDP templates. MapSAR will retain the last project folder you were in as default, so check to make sure you’re in the current project and path to your current project’s map templates.
b. If the problem persists (as I had happen once) it might be related to a .Lock problem (above). Save, Close out, delete other ArcMap.exe process then try again. Use the Unlocker utility.
c. If the MapSAR DDP Tool (the icon) still doesn’t work, the standard Data Driven Pages Tool Bar will still print your maps. For that workflow, see the Using GIS for Wildland SAR book, Exercise 8A, Print for how to do it.