With Boxcryptor 2.0 it's even easier then with Boxcryptor Classic.
It is enough to download the encrypted .bc file using your provider's web interface and place it in a temporary location. After setting up Boxcryptor and logging in, this temporary location must be added in the Boxcryptor preferences. Now you can open the Boxcryptor Drive, open the folder of the temporary location and access your file.
PS: Of even easier: You can use our plugin for Google Chrome which directy decrypts your file in the browser when you download it. You can get it here: https://chrome.google.com/webstore/detail/boxcryptor/ijmndaodmdjamfepoijpolhjddgfgmme?hl=de