Run a simulink model on new version

I have to run a simulink model created on Windows and Matlab R2014a in Unix environment and Matlab R2015a. When I try to run the model as is, I get the error "Invalid MEX-file". How to create the new MEX file? I tried building the model, it seems that the model builds successfully as the build command executes without errors, but the model does not run. Please Help!