Right-click on the game, left-click properties, and change your launch options.

Sorry if this is wrong but i recently downloaded the game for the first time and may have dodged the bullet that hit you. When you first launch the game you get a pop up asking to load normal game or with mods. on the same pop up is a tick box to remember choice. Maybe try clicking the cog button on the games home page on steam and selecting to load with mods from there.