For what its worth, My SWCG (Compu-pool CPSC24) shows my "ADD SALT" light when the weather is cold almost all the time. I don't get an exact reading just an idiot light. SWCG's measure the Salt level electronically and water temperature plays a big part in that reading. Once your pool water goes below 60° F most SWCG's stop producing chlorine or produce greatly reduced amounts of chlorine.

Where are you located and what is your water temp? Unless you are heating your pool, you might want to wait until the water warms up before adding salt. And as mas985 already mentioned, the SWCG is what has to be satisfied to work properly.