Creating a Vector Mask from a Path

Optionally, you can create a vector mask from a path as follows:

Make sure the required path is active (outline being visible).
In the Effects palette, double-click the "Vector Mask from Path" pictogram.
If necessary, invert the mask, adjust its size, etc. (more about editing vector masks).

NB Photoshop Elements doesn't allow you to draw on an existing vector mask. To create a mask from several components, you create a complex path from several shapes or selections and then convert it to a mask.