I set mine up a couple days ago and I believe it's the 6000XP serial number you use. Once you plug in that info it knows what to read. In the app, look just below the MONITOR tab and you'll see a box to check "SELECT STATION FIRST". I gave mine my name, the type in the inverter serial number in the box next to that. This should get it reading your 6000XP.
Also, when you go to the MAINTENANCE tab you will need to click on READ (next to the serial number) every time you go to this tab. I think this is for those with multiple inverters, so it knows which one you want to look at. I kept thinking something was wrong, it didn't seem to keep my changes. Turns out I was wrong, I wasn't telling it where to READ the info!
Mind you, I'm only a few days into this adventure, I'm still learning...