How do I get the z depth output?

In the global render settings, on the common tab, you can add the Depth Channel (Z Depth) setting to your renderable camera. Then on your Final pass output tab, you can change the rgba output to use rgbaz channels. That way, the z channel is included in your final pass.

If you'd like to have the z channel in a separate AOV, select your Final pass in the RenderMan controls window and then click on the outputs tab. Then add a custom output named "float z". Then you should add the Filter setting to the z channel and change it to zmin.  You'll need to make sure your output file format is a format that will support the depth data, such as 32-bit Tiff.  Once you have the z depth output file, you can open it in Nuke, Photoshop, or some other image editing program and then correct the exposure or normalize the image so that the pixel values are remapped between 0 and 1, then you should see the depth map values.