Whenever I try to board the PMDG 737-800 (using the profile supplied in the offline installer provided by Umberto), it doesn't allow me to. This is because it always displays the message of 'Open Exit 1, Open Exit 3',
That's because you are not using the GSX internal profile. You must still have your own configuration, one you tried to create when the airplane wasn't supported yet but, since it was made without any proper "base" config, it's showing all generic names like "Exit 1", etc, which are a tell-tale the airplane is NOT configured by the GSX internal data, otherwise you would see them labeled with their proper names and they will open/close automatically.
This because, your custom configurations always take precedence over the GSX internal one, and yours was made *based* on the still unsupported plane, nothing basically works.
Use the RESET button in airplane configuration page, and be sure it reads "Internal GSX database" there, and nothing else.