It's a mono speaker.
The DD1+ has a mono output.
So, connect the DD1+ L/mono Out to one of the Line inputs of the B112D.
The speaker will accept a balanced input on a TRS plug. It's unclear whether the drums offer a balanced output. A TS-TS cable will work. If you HAVE a TRS-TRS cable try that too. There's a slight possibility you'll get more signal ...
There is only one channel available on the powered speaker. So doing anything 'stereo' is a waste. You have L/mono out of the drum brain. That sums it all together. Use that, and plug into one channel of the Behringer. One signal in, one signal out. Simple, and using the simplest plug arrangement - mono to mono.
To make it sound really good, get another ...
Since you have one speaker and the Behringer L output doubles as a mono output (L/mono) use choice “2”. All you need is a single mono 1/4” instrument cable. The drum machine will output a mono signal if nothing is connected to the right output and a TRS type cable is not necessary since it is a mono output.