First, you need to login into your account on https://metamundo.co/ to access the files. If you haven't already swapped your MetaPortal mintpass, please see the instructions on how to do that here: https://help.metamundo.co/en/articles/6163300-how-do-i-redeem-the-metaportal-3d-nft-and-download-my-3d-files.
Download the "Decentraland optimised" file from your Collected NFTs under your profile.
Navigate to the Decentraland Builder: https://builder.decentraland.org/
Create and open a Scene inside the Decentraland Builder Editor
Create a custom package using the “New Asset Pack” in the end of the assets listing or use an already created custom package
Import the MetaPortal using a custom collection from Decentraland (drag and drop the zip file to the screen or select it from your computer)
Add The MetaPortal to your land
Select Land and then add The MetaPortal within the scene on your land. Then click update and you have successfully added your MetaPortal to your land.