to the top of your MATLAB function, and I think it will work in MATLAB. Basically, the compiler is expecting to be given MATLAB code to compile, but when it goes to try to compile Addi_mex, it finds the binary mex file, which it can't handle. If you declare it extrinsic, the compiler never tries to compile Addi_mex, just sets up a call to it.
However, for standalone code generation, ALL inputs must be in MATLAB or C. The compiler cannot deconstruct a mex file and emit C code. For this you wouldn't use your MEX file, rather you'd write a C library that is used both by your MEX file and your coder project. Then from your MATLAB function you would call into this library using coder.ceval.