This video describes and shows the process of lubricating a walking belt on most styles of treadmills. You can purchace the lubricant needed on:
Icons parts Store,
https://www.google.com/url?q=https%3A%2F%2Fwww.iconservice.com%2FCustomerService%2FcartAddItem.do%3Fpart%3D219168%26model%3DCSWEBSALES%26modelVersion%3D0%26company%3D11&sa=D&sntz=1&usg=AFQjCNHaStvDUwHvg4nXsgUQ0F4qUUbAJg

or on Amazon,
https://www.amazon.com/ICON-Health-Fitness-Walking-belt/dp/B0000EIF9S/ref=sr_1_2?ie=UTF8&qid=1473459751&sr=8-2&keywords=icon+treadmill+lube