As Martin said, the function prototype is a little misleading (or at least unclear), but depending on what version of CVI you have, the help file may actually specify that the arrays have to be doubles. CVI 8.5 and later mention this in the help file if you search for "matrixmul". You can also see it in CVI 8.5 and later if you right-click on one of the Matrix controls on the function panel for the MatrixMul function.
Eric B.
National Instruments