You get that error because laser's units make use of his weapons addon, and you need to declare it in the mission.sqm:
Go with windows explorer to your ofp\users\(nickname)\missions or mpmissions folder, then go to the name of your mission, open the mission.sqm with notepad, and at the Addons[], add a line somewhere in between the other lines with: "LSR_USWP",
with the " " and the , just like all the rest. If you put it at the end of the addons[] declaration you shouldn't add the trailing ,
See for yourself in the mission, its quite easy ;)