Ah, forgot that: use the float(var) instruction. I think this means you can't set them in the material definition, but have to do it somewhere else in your code:
material.skill1 = float(0.5);

This is from the manual:
Quote:

The skills are _not_ automatically converted to float. This has to be done by C-Script through float(), floatd() or floatr() instructions.



shader tutorial