I Have a Bosch KAD62V70AU fridge and trying to conct to the water line. Went to bunnings to get a connector and the bosch hose line is not the same size as the water line that the plumber out in. They sold me a plastic connector thingy.

Have put it all together now, but the connection drips now.

Any ideas on what I have done wrong and how to fix it?

Most Ice makers connect to 1/4" OD copper or plastic. ( I prefer copper) Here's a typical ice maker kit. (see image) check it out at , Eastman 48398 Ice Maker Kit
Good luck Tom