To be able to add a horse to the Derby, you need to click on the Add Horse button.

 

 

If you have a horse on your Farm, you will see a yellow arrow above it which indicates that you can place it in your Derby.

 

 

If you do not have any horses on your Farm, you will be prompted to open your Animal Barn to move a horse out.

 

 

 

 

After placing the horse, you will be redirected to the FarmVille Derby window where you need to click on the Add Horse again.