Hi Julian,
Sorry to hear you aren't having a lot of luck with your ML/NL Converter. It could be that someone gave it a static IP address before or it's not seeing DHCP properly.
I'm assuming this converter is 'new to you' but secondhand? You might find this document useful that is floating around online BLC_NL-ML Installation Guide Ver. 1.2.pdf I might be able to send it to you on email if you can't find it.
I think if you don't care for the configuration that will currently be on it, you're best to factory default it. to do this you use a paperclip or similar to push and hold the setup pin in on the back. if you push and hold you should see it will flash, you need to count the flashes until it gets to the 5th flash (factory default), release and then within 3 seconds press it once again - this is to confirm. the device will now reboot and reset to defaults. the login is admin and admin.
On your Mac I would recommend downloading Fing or Fing Desktop. this is a program that can easily scan your DHCP pool and show you what devices it can find. it should find the converter and tell you the IP address. Type that into your web browser and you should be good to go.
If you need any other help and I can help, let me know.
Thanks,
David