Employer will not pay after I got fired from work.

When you say that you were not paid, are you referring to money that you had already earned prior to being fired or severance pay?  If you are owed the money for work you had already performed before termination, your employer must pay you, and (assuming you had not previously committed to do this, in an employment agreement for example) cannot require you to do anything else as a condition of being paid.  In most states, there are very serious consequences for an employer who does not pay his employees.  However, if you are referring to money about which there is a genuine dispute such that the employer does not necessarily owe  it to you, or if it is money that the employer is offering as severance pay or in order to obtain a release of claims you may have, it can require you to sign a confidentiality agreement as a condition to receiving money.  In other words, if you are getting something to which you are not already entitled, the employer can require you to give up something (i.e. sign a confidentiality agreement) in order to receive it.  You should be aware that, even if you haven't signed a confidentiality agreement, you may be precluded from disclosing certain confidential information and trade secrets of your former employer.