Left rear bottom corner of the transaxle. (points toward the ground)
There should be a recessed plug that takes a hex key ("Allen wrench") to remove. Clean it out well before attempting to remove it, and make sure you tap the hex key in until it bottoms out. (more area to grip means less area to slip

)
If you happen to find a stripped hex, a Torx bit (I forget the size) can be driven into the plug and used to remove it.