A diode can go in either lead, but traditionally it goes in the positive, the silver band towards the device being supplied, in this case the looper.
What Jp was saying is that sometimes the diode is placed inside the device to be protected across the supply inversely connected, so that if you power the thing with the wrong polarity the diode conducts and effectively shorts out the supply, blowing a fuse and protecting it, only if there isnt a fuse, and there often isnt problems are going to arise.
Jp a looper is a sampler, you pressa button it samples audio, then when you next press the button it sstops ampling audio then plays what it has just sampled over and over, you can sort of record & play your own accompaniment as you go, very popular for street performers.