After installing Microsoft Expression Blend, you can enter a product key at any time during your 30-day trial period by clicking Enter Product Key on the Help menu.
A licensing dialog box will appear each time you open Expression Blend until you’ve entered a valid product key. The dialog box will contain the following two options:
I have a product key and want to license Microsoft Expression Blend Choose this option if you have a valid product key and want to enable full licensing rights to your copy of Expression Blend.
Continue using the trial version of the product The trial version does not require that you enter a key, but after the 30-day trial period ends, you will be prompted to enter a valid product key to continue to use the product.