URGENT!: Adding a new setting to the GlobalSettings section

Ok.  That clears a few things up.

Most of the settings found in the admin area are simple strings that correspond to some arbitrary value.  What makes this approach powerful for customization is that you are not stuck using a single convention.  Open up all settings and you can see the long list of settings that correspond to strings, boolean values, integers, etc.  This allows you to setup a configuration type setting in anyway you want.

For your example you could create a new setting and call it something like, "Display.ShowCategoryNavigation".  You would then set it's value to true or false based on your needs.  However, this will only register the setting you would need to wire it up.

A quick and dirty example would be to override the OnInit event in /modules/categorynavigation.ascx.cs.  Like this:

protected override void OnInit(EventArgs e)
        {
            if (SettingManager.GetSettingValueBoolean("Display.ShowCategoryNavigation") == false)
            {
                this.Visible = false;
            }
        }

You would also need to reference NopSolutions.NopCommerce.BusinessLogic.Configuration.Settings.

As you can see there are countless ways in which you can use admin settings.  All you have to do is wire up your logic in the appropriate place and call the SettingManager.

Hope this helps.

EDIT:  I just noticed you wanted to make this setting available under Global Settings.  The solution I have above has the setting only in 'All Settings'.  If you wanted to make it available under the former then a little more work would be necessary.  Basically what you would have to do is either make a new control tab and then wire a checkbox to your setting or just add a checkbox and the wiring in a existing tab.  NOTE: You will find most of the tab names have corresponding control names in /admin/modules.

What setting are you trying to read, and from where are you trying to do the read?

The stackoverflow response is inadequate.  It only covers how to add a setting to be visible on the Admin config page.
You need to also create the setting property in the CatalogSettings class (...src\Libraries\Nop.Core\Domain\Catalog\CatalogSettings.cs)

I recommend that you
1) in Visual Studio, open ...src\Libraries\Nop.Core\Domain\Catalog\CatalogSettings.cs
2) right click the last property ManufacturersBlockItemsToDisplay, and Find All references
Besides adding your new property after ManufacturersBlockItemsToDisplay, you will see the other places you need to modify

Do the same  Find All references in
src\Presentation\Nop.Web\Administration\Models\Settings\CatalogSettingsModel.cs
(this is what was referred to in the stackoverflow response)