DVDDecrypter didn't close my DVD's session -- How can I do it manually?

For some odd reason, DVDDecrypter didn’t close the DVD’s session or the disc.

How can I manually close the session and then close the disc?

I have Nero 6, RecordNow DX, and DVDDecrypter.

Well if I understand your question properly, with RecordNow there are options to do it. Check the guide in my sig for details.