Hi Brian,
I guess because I'm using Carrara Express...I don't seem to have it...or some of my tool symbols are turned off and I can't find anywhere to turn them on again?
If yu look at my assembly room shot above..you can see the tools I have to pick from.  I've clicked on everything I have access to..and there isn't a choice in the insert menu for spline either.

 

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[Stuart Chase]