MAMP Pro OS X Lion FTP - Cannot Find wp-content directory
To find a folder on your filesystem open the Terminal.app then run (omitting $):

$ find / -name "wp-content" -type d


Or since you're looking for the wp-content for wptest:

$ find / -name "wp-content" -type d | grep -i wptest


Alternatively you could look for MAMP's httpd.conf file, open it in your favorite editor, and locate the DocumentRoot path for the subdomain you're trying to access:

$ find / -name "httpd.conf" -type f


Take the path that is returned and paste that in to your FTP client's path bar, or use Finder to browse to that path.

As a recommendation, I think you'd be better served by not FTP'ing to your local machine unless you have some fancy reason for doing so. FTP by itself is insecure, deprecated and moving forward I'd recommend you enable Remote Login (under System Settings -> Sharing) and transfer files using SCP... though I'd just use Finder, or point your editor directly to the path of your development environment.