Unfortunately, the plastic on *some* of these LEDs is backwards, and we didn’t catch it before they shipped. I apologize for this; we probably should have noticed and included an erratum with the kit about it.
The correct solution is to install the LED as instructed, with the long lead into the square hole. However, the plastic may be backwards from how it is pictured.
I am so glad you posted this – I thought I was going crazy! I was *sure* I had put the long end in the square hole, but when I looked at which side was round, it was facing the wrong way and did not match the picture in your most excellent instructions. (how about posting a note in the online instructions?)