To access a 1228 x 681 matrix from a MAT file in C++, users need to understand that C++ does not have built-in support for matrices. Instead, they can utilize arrays or STL containers like std::vector to create custom matrix structures. For reading the MAT file, converting it to ASCII format is recommended, allowing for easier input using cin operations. Users may also need to implement their own matrix operations, such as multiplication and addition, since C++ requires custom operator overloading for these functionalities. Resources such as the Numerical Recipes website and MATLAB's save documentation provide further guidance on handling MAT files and matrix operations in C++.