If you are having problems with enabling the mod first of all make sure it's installed by right clicking on the game on your library > Properties > Workshop
1.First of all activate mod interface:

• Open the game
  
• Options -> Gameplay > Scroll all the way down until "Activate the mod interface" and make sure the box is checked.
  
  1.Open the game
  2. Click Play > Normal Mode (that will start a new run) Refer to image below:
  3. Click to fill the box with the name of the mod "1000x cells"
  4. Start new run
  Your "mods" options should look like this: https://imgur.com/a/6d8Bq0N
  
  What if I want to disable/stop using the mod?
  1. Unsubscribe from the mod on steam properties of the game
  OR
  click on the box that says none on the Mods on the game, should look like this: https://imgur.com/a/6d8Bq0N
  Always when enabling or disabling the mod, make sure that you start a new run (not a new save file) A NEW RUN.
  
  Hope it helps, comment if any issues or questions. Good luck on your runs!