Circular Convolution in Scilab
I can't tell about SciLab but if it is not implemented, you can always get the circular convolution by

IDFT(DFT(x1) * DFT(x2))

where x1 and x2 are your signal vectors and the multiplication is element-wise. If the signals have a different length, zeropad the shorter one to the length of the longer one. This assumes that you want to do circular convolution with a modulus equal to the length of the longer vector.