Ok, I see it now. I had gone into the "custom formula" editor for one of the columns I wanted to concatenate and didn't see the processor there. I do see it when I just click the "Add a step manually" button from the script editor on the left-hand side of the screen.
You can also use the "custom formula" editor to concatenate columns, by using the following syntax : Column_Name1 + Column_Name2 + " - " + Column_Name3