1. Start wireshark, of course!
2. Select Edit the menu
3. Choose Preferences..
4. Expand Protocols
5. Find IEEE 802.11
6. Check Enable decryption
7. Press the button below to add your keys. If you are using an older version of wireshark, just enter your keys bellow in the text boxes, don’t try to find the button, it doesn’t exist on older versions of wireshark. 
8. Just click OK