I did a new installation of the latest (v 1.3.28) MP on a PC and SPLINE_WAYPOINT isn't listed in the command drop down list. Further, if I check the Spline box before clicking on the map, it drops a waypoint at 0,0 Lat/Long.
The first installation was on a Windows 7 machine. I subsequently tried it on another 7, then on a Windows 10 technical preview PC. They all behave the same.
This only seems to happen on fresh installations. I use two Windows 7 laptops for flying and they were both updated to 1.3.28 and work fine.
I'm hoping someone else can try a fresh installation and verify whether it works or not.