I bought the PMDG 737-600 and 700 and he asks if as usual the code, I entered, but every time I enter the code you choose, and now say that "the maximum activation is reached"the question is, what should I do now, I can use the 800 and 900

Send a support ticket on PMDG site

As Fabo said, register at http://support.precisionmanuals.com and open a ticket. They will reset your activations for you.Also be careful to enter your 6/700 key and not your 8/900 key.