As the fold is the centre of a garment piece, you don't need to add any seam allowance - because there will be no seam there! The pattern makers could have given you the whole piece to cut out but folding it in half saves them paper and saves you having to cut all the way around when one half is exactly the same as the other (I hope that makes sense !)
You only need to add seam allowances where seams will be formed ie on cut edges.
I always have the notches cut away from the pattern and after I have tacked and sewn the pieces together, using the notches to match the different parts together, I simply trim them off and then press the seams open.
I've been dressmaking for about 40 years now, so if you have any more problems, don't hesitate to contact me direct (
[email protected])