Comments on your solution
Entering matrices
I think your solution is valid, however it could be frustrating for the user to keep providing entries if you have a lot of entries and you know you mistyped the first.
A nice function to have a look at is x_matrix. It provides an easy interface for editing matrices.
Functions
I also usually really prefer functions, it makes it much easier to reuse your code and validate and test small portions. Naturally in this problem the SciLab provided function max() should be used, but you stated you wanted to use a for loop.
Code example
Taking into account the above statements, here is a small working example. You could expand it to let the user first provide the matrix dimensions.
function maximum = findMax( numbers )
maximum = -1e99;
numberOfNumbers = length( numbers );
for i=1:numberOfNumbers
if( numbers(i) > maximum )
maximum = numbers(i);
end
end
return maximum;
endfunction
[result]=x_matrix("enter a matrix", zeros(5,5) );
foundMaximum = findMax(result);
disp( "Maximum is " + string( foundMaximum ) );