Firstly you missed the first three steps of Doug's instructions, particularly step one which identifies the location of both files!
The following describes the manual way to add FT8 to two mode files.
1. Open up your Log4OM's configuration folder (Help/Open Configuration Folder).
2. Find modelist.csv file in the folder. Make a copy of this file and rename the copy modelist_user.csv.
3. Open up modelist_user.csv with notepad or text editor. find a place in the file where you want the FT8 mode to appear in the list - like after FSK441... and insert a blank line.
4. Enter in that blank line the following FT8;FT8;FT8 Save the file and close it.
5. Now find the RSTMode.csv file (in same folder), make a copy of it and rename it RSTMode_user.csv. Open up this new file with Notepad or text editor, add to the 3rd line which begins with JT a "|FT" to the end of the text on that line...so the line now reads JT>JT|ROS|FSQ|MSK|FT
Save the file and close it.
6. Restart Log4OM and you should be able to manually log FT8 contacts, set the FT8 mode, and set the appropriate signal report.
Then you missed the next post in that same thread that pointed to the download of the updated files to avoid you having to modify the files yourself!
In the Log4OM Options 'Select config' tab there is a check box marked 'Enable expert mode' which makes it possible for you to modify the 'Modes' file from within the program - See pages 35 and 36 of the user guide.
Please bear in mind that FT8 was only released to the public about 3 weeks ago and at this time neither ADIF or LOTW have included the mode in their definitions and to my knowledge only Log4OM, HRD and DXLabs have updated their software to cover it.
Our user guide is no longer being updated on a daily basis because of the development of Version 2.0 but these types of amended instructions are posted as soon as possible to this forum and detailed instructions of how to update these files by either editing them or overwriting with the latest file versions is well documented on this forum as a search will reveal.