When I try to add custom dimlets using https://github.com/McJty/RFTools/wiki/Adding-a-Dimlet , but when I try and load into a world I get 'A fatal error has occured, the connection has been terminated'. I am in minecraft 1.7.10 using version rftools-4.23.

I need to see a crashlog. Also show me exactly how you added the dimlet

I dont crash, just disconnected from the world. Also I didnt even add a dimlet, just the userdimlets.json.
I have tried changing it to CFG and I can join a world with no custom dimlets, just tried adding a dimlet to see if it works

Well show me what you added then. I cannot diagnose what I cannot see

so you are creating custom dimlets and sounds like the world is timing out while loading.

I am using notepad++, but I just missed the first and last square brackets in the image to begin with.

I was being a complete idiot, sorry guys. I Havnt done this before and have managed to make it work. This can be closed. Thanks

what was the resolution. could be useful to others that might come across this

I just didnt put the first and last square brackets into the file,

Anyone who comes across this, make sure you dont miss anything like I did, it causes so much annoyance.