Ok, so:
You might notice that in the InfraWorks 360 2017.3 release (aka January 2017 release of InfraWorks 360), a single-click from your mouse won't suffice to select a design group to publish that model to. However, there's a simple fix to this (also, this issue will be completely fixed in our next release of Autodesk InfraWorks):
1) Instead of using a single-click operation to select a different design group to publish a new Model Builder model to, you can double-click to select a different design group. Or, instead of using your mouse, you can also use the Up/Down arrows on your keyboard to select a different design group.
2) Then, with the group drop-down menu still open, and the desired design group selected, press Enter on your keyboard.
3) You can now click Create Model to let Model Builder build the new model to your selected Design Group.
Note: This issue was mentioned in the readme for Autodesk InfraWorks 360 2017.3, which can be found here: http://help.autodesk.com/view/INFMDR/ENU/?guid=GUID-17DA2F15-7ABF-4ACC-9B63-24006A356897
Principal Specialist, Infrastructure