Adding books to library on Box.com

Hello,

A long time ago I exported my library to box.com

I uninstalled Calibre from my computer and everything related to the library is now in the cloud.

I am able to use the box.com library with Calibre Companion, see the existing books and download them. However I do not know how to add new books.

Can I do it from Calibre Companion or do I have to install Calibre again on my computer? I have tried the portable Calibre version but I don't know how to access box.com to import or add the new books.

Could you please help me? I am not a computer savy person, so a step by step help would be greatly appreciated.

Thanks!