WPA key is a loose term that could mean your wireless network "password", or it may be the actual "key" for your wireless network...a long, complex code consisting of letters and numbers.
I'm sure that you know your wireless network "password".
To find out more about your "key".....
Open Macintosh HD > Applications > Utilities > AirPort Utility
Click Manual Setup
Shift the focus of your eyes away from the window in the center of the screen and move them to the very top edge of your computer screen to the menu bar.
Locate the Base Station menu, then click Equivalent Network Password
Your "key" will be revealed to you. That may be the info the that the TV needs, but no guarantees, of course.