To download a world, first navigate to the FTB Worlds screen in the modpack.

Now select your FTB World and click on the Configure button. You will now be on the configuration screen for your FTB World.

Click on the Worlds & Backups button to go to the world management screen.

Here you will find a list of your available worlds. Select the world you want to download, then click on the Download World button. This will show a progress screen while the world is being prepared for download, once the world is ready to be downloaded, it will open a browser window and download your world. The world will be saved as a zip file to your downloads folder.