The only trial version Adobe offers is for Acrobat XI (11), not 8...
Regarding the console, after entering your code, select it with the mouse and press Ctrl+Enter to execute it.
But I found Acrobat 8 on this page:
And thanks for the Ctrl+Enter tip.. This has to be done every time or can I enable somewhere that the entered commands/code are executed just on Enter?
If you have a numeric keypad on your keyboard, you can press just that Enter key. The regular Enter key is needed so you can type a new line.
But why would you install a trial of something you will not, later, be able to buy? Acrobat 8 is not actually offered for trial purposes, only for people to reinstall who had it before.