Acrobat XI pro activation

It should do so automatically, if you have activations left. If you don't you'll need to contact Adobe directly to get them to reset the activation counter. They might tell you that that version is not supported any longer, which is true but not relevant. You have to insist they reset that counter, or you won't be able to use it. Ask to speak to a supervisor if they refuse to help you.

To contact Adobe directly:

https://helpx.adobe.com/contact.html
https://helpx.adobe.com/contact/phone.html

Beware of people contacting you via the forums' messaging system pretending to work for Adobe! Only those with an "Adobe Employee" tag under their name are legit. Also, Adobe will never offer to contact you via Skype, or use an email account that's not under the adobe.com domain...