Makes sure you connect the Red and Black wires coming from the CELL and not the flow switch.
Also, make sure the wires are clean as they can get corrosion and cause a poor connection.
I am not sure how long the wires have to stay connected before the cell recognizes that the "switch" is closed. It might be instantaneous, or it might take a minute or so.
You say the light stays on. Well, the flow light should always be on.. It should either be red, for no flow, or green for a good flow. It could be as simple as the LED is bad and won't change color. If that is the case, the cell should still make chlorine and it should still report a good salt level.
As a test, set the output to 50% and see if the "Cell" light is on for about 2.5 minutes and then off for about 2.5 minutes. If true, this means the cell is making chlorine.