I'm not sure what you are saying. Do you mean that:
X = 10+randn(16,16);
X = uint8(X);
Does not result in integers for the output in X?
The error message you report ("Undefined function 'mrmr_mid_d' for input arguments of type 'int8' ") just sounds like MATLAB cannot recognize the function, mrmr_mid_d.m.
You need to add the folder containing that function to the MATLAB path. You can launch
to add that folder.