How to connect the planronics plx500 to the phone? what is the use of each one of the bottoms?

Unfortunately finding life on another planet would be easier than locating an ancient (electronic years) PLX-500 user guide. However, I happen to have one of those ancient writings (no, it wasn't in a cave at the Dead Sea), albeit a bit tattered and yellow, so I'll try and give you a rundown of it's various features.
The top buttons for headset/handset and mute, and listening volume wheel are pretty self-explanatory. Now let's talk about connecting the PLX-500 to your phone and the other amplifier features.
1) Disconnect the telephone handset cord from telephone base and insert the PLX-500 modular plug into the handset jack on the phone. You then insert the coiled handset cord into the back of the PLX-500 base (there's only one place it can go into).
2) Plug the cord from the headset into the jack in the front of the PLX-500.
3) On the back of the PLX-500 base there are two switches (1-2 and 3-4). These are used to configure your PLX-500 to your phone (most electronic single and multi-line phones, such as Panasonic, will use setting 1 and 4). You will have to configure the PLX-500 to your specific phone. Having connected your PLX-500 to your phone, put on your headset, press the headset button on the base, lift the handset of the phone cradle and listen for a dial tone. Adjust the switch settings until you hear the best dial tone.
4) Remember that your PLX-500 uses batteries located under a cover on the bottom of the base (some phones will work without your PLX-500 needing batteries, check configuration before installing batteries). Speaking of the cover, there is a small plastic screwdriver attached to it that you use for adjusting the volume of your microphone. That adjustment is located on the back of the PLX-500 base next to the configuration switches.
Hopefully this gets your PLX-500 up and running.

Good luck from Headsets Direct!