I created the file using pico BitcoinDark.conf and pasted in the text in OP, substituting the rpcuser= and rpcpassword= with the ones it suggested. Also changed permissions to 700 as recommended. Next time around, I got the same message.
Put your BitcoinDark.conf in .BitcoinDark in your home directory.
That directory doesn't seem to exist.
Edit: found hidden file with ls -a
Steep learning curve for me today.
cd
git clone https://github.com/jl777/btcd
cd btcd/libjl777
./onetime
cp randombytes.o libnacl.a ~/btcd/libjl777/libs
cd ~/btcd
./m_unix
./BitcoinDarkd
Thanks. Why? What goes in the SuperNET.conf file and where?
Also, I didn't realise you needed a NXT wallet running at the same time. Any particular requirements for that?