I'm sorry, but it is currently not possible to use any web application (e.g. MS Excel Web App, Google Docs, etc.) to open or edit encrypted documents. Main reason is that those web applications need access to the plaintext - which is not possible for encrypted files and also not wanted (why encrypt then?).
So if you want to use web applications like MS Excel Web App, using client-side encryption is currently not possible (not just with Boxcryptor but with any client-side encryption software).