My whirlpool dehumidifier freezes up !! whats the problem?

Your dealer who looked at it was almost certainly right in his diagnosis. "Freon" was originally a trade name for a limited range of refrigerant gases (the stuff that's pumped around in the pipes of dehumidifiers, refrigerators, freezers and most other coolers), but the name is often used colloquially to refer to any refrigerant gas.
When it's manufactured, each machine is filled with a measured quantity of refrigerant gas. This can leak out slowly over time, and freezing up is the common sign that this has happened. Most refrigerator engineers should be able to refill your machine and check for leaks at the same time.