I think he might have the same problem as I do: the custom model file picker dialog refuses to open any folder by claiming that it doesn't exist despite being listed right there in the dialog.
So, you need to create real versions of all of the folders in the /tf/ directory in order to get HLMV to recognize the virtual folders of the same names in the VPK files.
If this is your first time logging in after the migration (Feb 8, 2022), you must reset your password to log in. Follow this guide if you're having trouble