After installing Microsoft Expression Encoder or after clicking Enter Product Key on the Help menu, a dialog box opens that contains two options:
I have a product key and want to license Microsoft Expression Encoder Choose this option if you have a valid product key and want to enable full licensing rights to your copy of Expression Encoder.
Continue using the trial version of the product The trial version doesn't require you to enter a key, but after the 30-day trial period ends, you will be prompted to enter a valid product key to continue using the product. If you do not enter a product, the application reverts to Expression Encoder Express, which is a light version of Expression Encoder.
If you experience any problems with entering your product key, please contact Microsoft customer support.