Instructions
Purchase a wardrobe or dresser for your Sim's home. It does not matter which type you purchase or how expensive the item is, however expensive wardrobes and dressers have the bonus of boosting the environment rating of the room.
Place the wardrobe in an area where it is easily accessible. Make sure there is room in front of the dresser so that your Sims can use it.
Switch to the Sim that you want to dress up in a bridal gown and click on the wardrobe.
Select "Change Outfit" to change what your Sim is wearing at the moment. With this option, your Sim will wear the gown only until she changes clothing again, like after taking a shower or going to work. Select "Plan Outfit" and choose "Formal" if you want to change your Sim's default formal outfit to a wedding gown. With this option your Sim will always wear a wedding gown for formal situations.
Select a bridal gown for your Sim to wear from the available options. Bridal gowns are located on the outfits tab in the wardrobe menu. Use the customization tools to alter the appearance of the gown if you so desire. Save your changes and exit out of the menu.