Your license allows you to have the software installed and activated on two machines. So if you only had the one installation you still have one activation available to use. Just install the software and use your serial number to activate. If you originally purchased the software as a download thru Adobe then you might be able to find a link for the software thru your Adobe account online. The download is also available thru the following page...
Acrobat XI, X - http://helpx.adobe.com/acrobat/kb/acrobat-downloads.html
If you registered the software your serial number should be available thru your Adobe account online.
To locate the serial number:
For the machine that can no longer be accessed, you can still have Adobe Support take care of deactivating so that you again have the activation available for use elsewhere. If you contact them thru chat they should be able to do that for you. If they say they cannot, then try to get someone else because they are the only ones that can and are supposed to (some incorrectly think they are not supposed to).
For the link below click the Still Need Help? option in the blue area at the bottom and choose the chat option...
Serial number and activation chat support (non-CC)
I've tried that, but when I try it, the program says that the license is not valid.
The first computer was a Windows, and the new one a MacBook...
Have I to purchase a new license for the Mac?