You get you registration code via email. It consists of two strings - user name and code itself. It looks approximately like this:
Regname: John Smith
(Note: This is not a valid registration code, and code format may differ.)
You should copy the registration code to Able MIDI Editor. Start the Editor, then press "Enter Code" button. Use "copy-paste" from the letter you've got - you should enter exactly the same name and code. Also, make sure that you didn't enter any additional space symbols before and after the registration code and your name:
After filling in the code and name restart Able MIDI Editor. Registration allows using the Editor after trial period and removes the startup banner. To make sure you have successfully registered Able MIDI Editor choose "Help->Register!" menu item after restarting. In registered version "Already Registered" message box will be displayed.