MS Office Prof Word and E-postage

[catsears]

In working with Word 2007, I want to put e-postage (stamps.com) directly on my envelopes. I follow the directions, click on the e-postage or try to put a checkmark in the add postage, before printing envelope, but it's not allowing me or giving me the option that says it should.

What am I doing wrong? I have stamps.com already installed on the computer.

Thanks