No worries, I can walk you through this.
When you use the add media button you'll need to set one image as the "Featured" image, this will become the main photo for the vehicle and is the photo that is displayed on the inventory result pages (like the 'cars-for-sale' page).
To add more than one photo to a vehicle you'll need to create a Gallery. You do not need to 'insert' the Gallery, you just need to create it and upload the photos to it.
The latest version of WordPress (3.5) made some serious changes to the media tools and unfortunately it affects the way photos are added to vehicles, so the problem you are having is very understandable. In the near future we'll be making some changes to the PlugIn to take advantage of the new features in 3.5 which will make this a lot easier to understand and work with.