Hello timgreenwood,
if you're using the FREE-version (where only one location is allowed), you must first deactivate iCloud and activate Dropbox instead.
To do so, please go to the Boxcryptor settings, under "Locations", uncheck iCloud and check Dropbox (you have about 15 seconds to do this, otherwise the "autodetect" function will enable again iCloud).
Remember, due to autodetection, is it not possible to completely remove iCloud.
Best regards,
Yasmin