Okay I'm unsure of why you need to force update, unless the guide instructed you to. Open the launcher and go to the Options tab. Now click on the big button that says 'force update?' It should remain pressed. Switch back to the Modpacks selection screen and click on the pack you wish to 'force update' and then the Launch button. This will re-download everything. It will give you an option to save your configs if you like, but it is not required in this instance as you are still starting a new world.
