I'm assuming you clicked "Don't show this again" and you want it back, and are suggesting an achievement to go with it.
When they get achievements added, I don't believe they would add one for something that can only be completed in Early Access, as anyone who buys the game on release day wouldn't be able to 100% the game.
If you've disabled it, you can re-enable it with these steps:
Go to C:\Users(YOUR USER)\AppData\Local\Tower\Saved\Config\WindowsNoEditor
(You can get here by typing %appdata% into windows explorer)
Find a folder named "Game.ini", open it
Ctrl + F and look for "EarlyAccessNotice".
Change "True" to "False"
Save and exit, and when you launch tower again you should have your clicky-catsack waiting to be pet!