Why Matlab/Simulink can not find the S-Function file

I have a Simulink model with a user-defined S-Function block was built in MatlabR2010a, Windows x86 PC. This user-defined S-Function was written by C language. I want to run this model in a new release of MatlabR2015b, Windows10 x64 PC. Now I can open this model in MatlabR2015b but couldn't run it. When I run the model, it has an error "Error in S-function 'AAA/BBB/CCC/xxx': S-Function 'xxx' does not exist". Although the C source file of S-Function is in the same folder with the Simulink model file and I also have right click this S-Function block and in Function Block Parameters windows I then click Edit button, it still show C source file in Matlab Editor windows, but the model can not run. In this C source S-Function file, some Chinese characters in comments (after % character) are not show clearly in Chinese and I can't read these. Now I don't know how to solver this, please tell me how I have to do! Thank you very much!