I had a hard time to figure out that I had to click the cell to select what type of expression I'd like, that in my case was a decision table.

I right-clicked, I double-clicked the "Decision-1" header and took me a while to just click the cell that says: Select expression.

A very quick win for a very short time would change the message to something like: "click here to select expression"... but likely that a better interaction is needed to replace the current empty cell.