Are you creating your data files using DVDFAB software? I ask because:
If the target output from DVDFAB is the DMS, it should automatically create an nfo file for you.
Under Settings->Metafata, there is a check box that will force creation of nfo files regardless of the target.
As per Deadeye, the DMS itself will only create an nfo if you use the "fix match" function. The nfo files are a one time read by the DMS to assist the scrapper with retrieving the correct meta information from TMDB. Once it does that, the information is stored in an internal database and that database is used to display the information on the poster wall.
I believe you can force the DMS to re-read an nfo by using the WEB interface to delete the current entry on the poster wall. (Be careful with this as the delete function will also delete the video file along with the meta info. Rename the video file to prevent that). Once the info is delete, then have the DMS do a scan (complete library rebuild should not be needed) and it should re-read any nfo in the same direcftory as the video file.
I have tried manually editing nfo files and met with limited success. I do not believe the DMS has the entire nfo specification implemented. Some tags work, some don't.
Personally, I don't use the nfo files very much. I found the scarper in the latest firmware drop works quite well, assuming you are using the standard file naming conventions. I get 1 or 2 files that fail to scrape correctly per hundred files added, and so only use the nfo file when I am forced to do a "fix match".