The error message you quoted is reported by the 2.5D machining operations such as pocketing and profiling when a 3D mesh is selected.
These operations are designed to work with 2D curves.
CamBam can work with 3D meshes using the 3D or 'Bas Relief' machining operations, as described here:
and some notes on the very latest features such as front/back and mold making here
3D support is relatively new compared to the 2.5D operations and better documentation will be appearing soon.
I hope this and good luck!