How to configure TransactionManager programmatically

I'm having an application here based on Spring MVC. My workmate (btw he's not here) configured it programmatically and everything seems to work, except for the TransactionManager. I have never configured a Spring web application like this and I have no clue what to do and also can not find any documentation about how to configure a webapplication like this.

I'll just show you the 'AppInitializer' and the 'EntityManagerConfig'.

AppInitializer:

public class AppInitializer implements WebApplicationInitializer {

@Override
public void onStartup(ServletContext servletContext) throws ServletException {
    AnnotationConfigWebApplicationContext context;
    ServletRegistration.Dynamic dispatcherServletRegistration;
    FilterRegistration.Dynamic encodingFilterRegistration, compressionFilterRegistration;
    Set<SessionTrackingMode> sessionTrackingModes = new HashSet<SessionTrackingMode>();

    sessionTrackingModes.add(SessionTrackingMode.SSL);

    context = new AnnotationConfigWebApplicationContext();
    context.setServletContext(servletContext);
    context.scan("de.devbliss.doc");

    servletContext.addListener(new ContextLoaderListener(context));
    servletContext.addListener(new Log4jConfigListener());

    dispatcherServletRegistration = servletContext.addServlet("main", new DispatcherServlet(context));
    dispatcherServletRegistration.setLoadOnStartup(1);
    dispatcherServletRegistration.addMapping("/*");

    encodingFilterRegistration = servletContext.addFilter("encodingFilter", CharacterEncodingFilter.class);
    encodingFilterRegistration.setInitParameter("encoding", "UTF-8");
    encodingFilterRegistration.setInitParameter("forceEncoding", "true");
    encodingFilterRegistration.addMappingForUrlPatterns(null, false, "/*");

    compressionFilterRegistration = servletContext.addFilter("compressionFilter", GzipFilter.class);
    compressionFilterRegistration.addMappingForUrlPatterns(null, false, "/*");

    co