You don't have to do anything to make the PAPI lights work - but you have to be in the right place to see them.
(and x-plane only has 4 light PAPI even when the field may have a 2 light PAPI listed)
1) start a flight in your favorite aircraft at "GA SOUTH" on BTR.
2) press C to exit the plane, "bump" the up arrow key briefly so you can see rwy 4R there on your right
3) use the E key to spin to the right and face down the taxiway next to 4R
4) use the left arrow along with the Q and E key to move left to be looking down the runway centerline of 4R (the shorter rwy)
(You will see four light PAPI on the left just beyond the touch down line, probably four reds at this point.
Remember - X-Plane only has 4-light PAPI so when airnav says 2 light PAPI you will probably see 4 lights)
5) Use the Up arrow till the lights turn white, then down arrow again to see them turn all red again.
The center of the 3 degree precision approach is when two whites and two reds are showing.
All red, you're dead - All white you're high as a kite.
The airnav for BTR states 4L also has a 2-light PAPI, but it looks to me that the scenery designer implemented VASI lighting instead. same principle, all red you're dead, red over white you're alright, All white you're high as a kite.