If you use one stats code for all of your landing pages you are best off setting the global stats code. That stats code will automatically show across all of your landing pages.
In the event that you need to have different stats codes per landing page you can set this value and it will show that stats code for the landing page.
To set a stats code per landing page simply go the "Edit" mode for the landing page you wish to edit & click on the "Site Settings" menu. Paste your code under the "Site Specific Stats Code".