It is physically inside the StarCraft II CASC archive. CASC is a very complex archive system where data is stored in massive data files and hash based lookup systems are used to locate where the data is. Since Heart of the Swarm they migrated to using CASC instead of MPQ so mods no longer seem to have a physical manifest, instead they are inside the CASC system. All current Blizzard games use CASC.
You can open the CASC archive using a third party CASC browser. You can also use the editor to extract files from within the CASC system however this is likely much more limited than what a CASC browser shows.