That 'warning" is just so you don't connect the black tank flush to a hose and not open the black tank valve. It's to prevent you from filling the black tank via the flushing system and backing up black water into the inside of your TT.
When you say you couldn't get it to work - what are you expecting to happen? You should simply hear water spraying into the black tank and it may not be all that loud.
If you don't hear anything nor see any clean water coming out of the black tank hose after it's emptied and you keep the flush hose running it can be one of two things.
1. You don't have enough water pressure in the Back Tank Flush hose to spray inside the black tank
2. You have a faulty check valve on the Black Tank Flush
See, there is a one way valve on that flush line so that you can put water in to the tank but no "black water" i.e. sewage can get back up through that line. It's called a check valve. It's not super common, but it's not impossible for that valve to fail and block all water flow.
One way to tell is if you have the hose on full power on the flush line and no water is entering the black tank (turn the hose on and go inside and stand on the toilet flush and see if you see water coming in - or if you hear water coming in.
Being new to travel trailers everything can seem like a big deal. It's not. Relax.
If the BW Flush is truly not working you can have any WBGO dealer fix it under warranty assuming it's still under warranty. Even if it's not, it's not a big cost.
Also, while having a black water flush is super convenient it isn't 100% necessary and it wasn't all that long ago that no RV had such a thing. So, again. relax.
PS. Use a separate hose for the black tank flush hose - don't use your white fresh water hose. It's not sanitary to use the fresh water hose for this purpose.
2017 Winnebago Adventurer 37F
2016 Lincoln MKX Toad