During analysis of the log file Visual Studio needs to be able to find the symbol files (.pdb) that were generated when the application was built. The application can be compiled using the "release" mode as long as symbols were included.
When you open the log file you need to make sure that Visual Studio knows where the matchng symbol files are located. You can set this location in Tools -> Options -> Debugger -> Symbols.
The symbol files contain the data that matches the compiled binary code back to source files.