EAPoL is an authentication protocol which is also used in WPA/WPA2. Its utility is to authenticate a user and establish a shared data from which the future encryption key will be derived.
EAPoL payload (i think you meant the handshake messages) are transmitted via Wi-Fi (in our case). The details of the actual payload content can be found here. So in order to get it, you need to:
- Ensure your wi-fi card supports monitor mode (e.g. listen to all the traffic nearby).
- Start to sniff that traffic (e.g. with help of WireShark and aircrack-ng tools)
- Deauthenticate a connected user to force him in re-authentication. That's when that handshake will occur and since you are listening to traffic, you'll be able to see and save it.
Here is a step-by-step guide into cracking WPA2 through EAPoL handshake. You should also know that in order to perform the crack you'll have to brute-force password which means you might succeed only if your password dictionary will contain a right one.