Issues with integrating custom C Code in Simulink

Sorry if there's a similar question already, I wasn't able to find one all morning. I am trying to integrate a custom code snippet into Simulink. I followed the instructions in the documentation as follows (I understand this problem could be due to my lack of mad C programming skills but I'd really appreciate it if someone would point me in the right direction.)

//custom header file saved as mycustomcode.h

extern double mycustomecode(double , double );

//custom c code file containing the function definition saved as mycustomcode.c

#include "mycustomcode.h"

double mycustomecode(double inval, double constval){

double outval;

outval = inval + constval;

return outval;

}

In my Stateflow Chart I have a default transition where I have the function call :

{SF_Out = mycustomcode(SF_In,constant_1);}

The Output variable and the two input variables, I have defined in the model explorer.

From within the SF Chart I go Tools>Open Simulation Target > Simulation Target > Custom code and under Include Custom C Code in generated : Header file :

#include "mycustomcode.h"

and under Include list of additional : source files :

mycustomcode.c

I try to Build or Update Diagram but get the following problems :

c4_test_log_in_sf.c(182) : warning C4013: 'mycustomcode' undefined; assuming extern returning int

and upon linking :

c4_test_log_in_sf.obj : error LNK2019: unresolved external symbol _mycustomcode referenced in function _sf_c4_test_log_in_sf test_log_in_sf_sfun.mexw32 : fatal error LNK1120: 1 unresolved externals NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin\link.exe"' : return code '0x460' Stop.

Someone please help me :)