There is no enable, you just use it so select it and browse to the file you want to attach.  If you don't see the attachment button then make the screen bigger and it should appear or find it in the upper right of the screen.  

I can see the button but is not enabled. I click on it but does not work.

You have to select a transaction and it should become active.  

Does not work.

Have you saved the transaction? I found this in the Help:

"When you are adding an attachment to a record or transaction, you must save the record or transaction before you can add an attachment."