I was unclear above. For the benefit of anyone else trying to figure out how to use BoxCryptor and having the problems I was, just follow the following steps. Hopefully this will help.
1. Download and install BoxCryptor to their suggested default drive. Allow it to open. Amoungst other things this will put a BoxCryptor shortcut on your desktop. If you click this icon nothing will happen as the programme is already running.
2. In Dropbox creat a new empty folder called something like "BoxCryptor"
3. Right click the BoxCryptor padlock icon in your notification tray and select preferences
4. In preferences select the path to your newly created folder, nominate an unused drive letter like Z for your virtual drive and nominate a password. (I chose not to select "remember" so that, as long as I remember to exit BoxCrypter after every session, I have to start BoxCryptor each time I want to access these files and am asked for this password everytime.)
5. Now go to your computer and you will see the virtual drive is mounted alongside c: etc.
6. Drag into the virtual drive the files you want encrypted in your dropbox.
7. You now can access the unencrypted files from this virtual drive. If you look at them in your dropbox they will be undecyferable.
8. I now go back to my notification tray and exit BoxCryptor so that no one who has access to my computer can access the file without knowing the password. (If you dont do this, though the file is encrypted from any other computer, if someone gets acccess to this computer they can view the file in the virtual drive without knowing the password).
9. Having exsited BoxCryptor you will see that the virtual drive you created no longer shows in your computer.
9. When you need to access the encrypted folder click on the BoxCryptor shortcut. It will prompt you for your password. When you enter the password the virtual drive will again show in your computer alongside C: etc. When you view the file from here it is unencoded and easy to read and ammend.