Drawing a curve with the Pen tool

To create the Back and Next buttons we are going to draw a curve using the Pen tool.

Choose the Pen tool.
Set the Pen tool's stroke properties to:

Make sure it also has fill set to None.
Click to start the path roughly in the position shown below:

A blue curve point will be created where you click.
This next step involves clicking and dragging the pen tool to create the curve. Position the Pen tool as shown below:
Click and drag the Pen tool to the left to create the curve:

You may have to go off the edge of the image to create the required curve.
Choose View > Guides > Show Guides.
Hopefully you still have a vertical guide spanning the centre of your image. We are going to use the centre guide as a reference.
Choose the Subselection tool.
Click the Subselection tool cursor once on the first curve point you created with the Pen tool.
The one at the start of the curve you just created. The curve point should now be solid blue.
Alt-drag the curve point to the right to modify the curve.
Notice you have just added curve point handles to the first curve point.
Use the Subselection tool on the curve point handles of the curve to adjust it so it looks similar to the one below:

Make sure it doesn't go over the centre guide.
Name this new path Button Line.