You can import a Figma file by copying & pasting the Figma URL link. Before that, you must connect your Figma account to Mason.
Connect Figma to Mason
Click on the [+] icon on the Homepage and choose a use case

Now click the design file icons on the top-right corner of the Page Design Panel

On the modal that appears, click on Connect button

Log into Figma using your credentials and grant permission to Mason to access your Figma account.
Import a Figma file by copying & pasting the Figma URL link
After connecting your Figma account, copy & paste the link of the Figma file you want to import and click on the Upload button.

Select the artboards you want to import and then click on Add artboards button. You can import all the artboards or a selected few as per your requirement.

Depending on the file size, it can take a couple of seconds to finish the import process