I could not make out the brand name on the spreader in the picture. It looks fairly new. Is the web chain ground driven or PTO driven? If the web is ground driven it should be easy to figure out from the instructions often bolted somewhere on the side of the bed.
If you can find a tag with a make and model number you should be able to get set-up and operating instructions directly from the manufacturer or by searching the internet for the instructions or a dealer's name.
From the picture it looks like you've already used it, why bother asking now? -- Just kidding!