OctoPrint mjpg-streamer configuration
The commands in your documentation only work with the Raspi camera.  If that's the camera you're using, you can use the tools mentioned from terminal on your Raspberry Pi, or over SSH.  

If you don't have SSH setup and are using your Raspberry Pi with a keyboard and mouse, you can access the command line by pressing ctrl+alt+f1  remembering that ctrl+alt+f7 brings back your GUI.

Setting exposure for Raspi camera

First navigate to the folder you installed mjpg-streamer.  If you installed it in your home folder:

cd ~/mjpg-streamer/mjpg-streamer-experimental


Then use the input_raspicam.so utility to set the exposure using whatever preset you'd like.  

./input_raspicam.so -ex night


If you don't have a Raspi camera

I'm currently using a Logitech HD Webcam C310 with OctoPrint.  I'm pretty sure setting the exposure for that is a pretty standard operation and you shouldn't need to install anything.  v4l2 (video 4 linux 2) comes installed with Raspian. 

To set the exposure for a Logitech camera (possibly others too) first enable manual exposure mode (1 for manual, 0 for auto)

v4l2-ctl -c exposure_auto=1


Then set the exposure value with

v4l2-ctl -c exposure_absolute=100


replacing 100 with whatever value you'd like.  The values start at 1 and have a maximum value of 10,000.

These commands can of course be run over SSH.

You can also run 

v4l2-ctl --list-ctrls-menus


to see all the options for your camera.

Here's a screenshot of my ssh session and my OctoPrint interface for context: