Unfortunately, the Chrome plugin has been discontinued.
If you have just a few files (and have some only from time-to-time) you don't have to install Google Drive Backup & Sync.
Just download them (the encrypted .bc files), put them into a location defined in Boxcryptor (internal/external disk, etc.) and you can open the files.
Remember: if you change/edit the files locally, you have to upload them manually back to Google Drive if you are not using their sync software.
Or, you use our "Portable" version: https://ptc.secomba.com/api/boxcryptor/windowsPortable/latest