For everyone else, I believe decrypt is greyed out if all the required ducks are not lined up. Content sharing needs to be on, the recording has to have been indexed by the DLNA system (usually a short delay after the recording completes, but not with the Humax in standby), and if it's HiDef autounprotect has to be installed.
I see that to use auto-unprotect the box must come alive during the night. I accept that it may be hard work for the system but if, during the day, it is on (not in standby), but not being watched, it would be ideal to be able to decrypt just the file needed - I don't need all decrypted, just the odd one maybe in a month. However, I am grateful for the facilities as they are
However, for your requirements, install autounprotect and create a folder "Decrypt" (or something), use the WebIF to mark it for auto-decrypt, then all you have to do is move any recordings you want decrypted into the Decrypt folder.