I'll activate Nintex from the central administration.
Nintex Workflow Management Application --web Activation. And I turn, but get error.
Warning: Unable to add the application pool account to the Nintex Workflow database.
application pool had in the farm bill and change it to another account, did not work.
never had this error myself but did you make sure the nintex Database has been created properly? You can re-create it very easily by slightly changing the database name in the central administration. Also make sure you have the latest database version. If a newer version is available you will see this in the Nintex Workflow Database Setup in the CA.
Does the account you use to perform this operation have sufficient permissions? Did you start your browser as administrator? I tend to have silly side-effects sometimes when I forgot that.
Additionally you should check the ULS logs to see if you find some useful information there as well.