Unfortunately MATLAB does not (currently) allow defining default values in the function definition line itself (nor does it allow arguments to be specified by name, only by position). To define a default value you can use the input parser class (I find this slow and pointlessly complex), or simply use nargin like this:
function out = addnum(mat,n)
if nargin<2
n = 3;
end
out = mat+n;
end
For more multiple optional arguments you should consider using a structure or name-value pairs: see my FEX submission num2words for an example of this.
