This option to manage archives is not available in Mod Organizer 2 so you can ignore this part of the instructions.
The archive you created has the textures folder in the root correct? If not, you'll need to expand each folder until you find Textures, right click the parent folder, and set as data folder. Also make sure you are using at least Mod Organizer 2.1.13. All this is covering the basics just in case...
If you used 7z or rar to create the archive, you might try using zip instead (e.g. right click, send to, compressed folder) to see if this makes a difference. I seem to recall 7z added a new compression algorithm recently and it may not be supported in Mod Organizer yet.
If all else fails, you can just open the mods folder, create a mod folder (say Texgen Output), and drag the textures folder into this folder. If Mod Organizer is already running, right click in the left pane somewhere and select Refresh. You should see the mod at the bottom of the list.