Alejandro,

What happens if you do the compiler verification? (Open Preferences by pressing Cmd + , or from the SystemModeler Menu select Preferences. Select Global->Compiler and press "Verify")

Try opening XCode. Make sure that the license agreements for XCode has been accepted (this has to be done manually when XCode updates).

It sounds like there's a problem with you XCode setup. Can you compile a simple "Hello World" program? Create a file test.c:

//
//  test.c
//

#include <stdio.h>

int main(int argc, const char * argv[]) {
    //Print to console:
    printf("Hello, World!\n");
    return 0;
}

Open Terminal and navigate to where you saved the test.c file. Run the following commands:

$ gcc test.c
$ ./a.out

The following should now print in your Terminal window: Hello world.