Close! Actually, htdocs is the root folder of the server that MAMP creates for you, so the 'Applications' and MAMP part aren't included in your localhost address.
If your WordPress files have all been copied into htdocs, then the address for the login page is:
http://localhost:8888/wp-login.php
If you put a folder called /wordpress/ in htdocs and then copied all your WordPress files there instead (for tidiness, or so you could have multiple installs for testing, etc), the address would be:
http://localhost:8888/wordpress/wp-login.php
Try that and let us know how it goes!