Configuring Parameter of an element

I am trying to create a simple workflow.
1. read text file
2. test converter
3. write outfile

Earlier I tested read text file and write outfile, it works.

However, when I add test converter, I cannot find a place where I can put in a script. I don't know where to click to put in a script? I am not sure if this is a bug or I have done it wrongly?

Thanks.