I believe fixdt
is for creating fixed-point data type signals in Simulink, rather than MATLAB. You would then define your signal to be of that data type as shown in http://www.mathworks.co.uk/help/simulink/ug/working-with-data-types.html#f14-90565.
If you want to create fixed-point objects in MATLAB, you probably want to use fi
instead. There are casting examples in the doc too.