Once the cake is covered in mmf I would like to make apples out of fondant and place them around the bottom layer of my 3 tiered cake. Do I have to allow them to dry before attaching them to the cake? or can I just stick a toothpick into the back of the apple and stick it into the cake?

can do either.

You can add some gumpaste to your apple to help it dry faster and make it easier to handle.