drush can't find the command

[fab01]

Hi,

I'm trying to install the tool just following the guidelines into the read me file.
There are several things that are not clear and, probably for this reason, the tool doesn't work at all.
I am working on Mac OS X and my Drush CLI is correctly installed and working fine ( for example with Omega Sub-theme generation ).
The READ ME file jumps from point 3 to point 6... ( Maybe I am missing something on point 4 and 5? )
I didn't understand where to create the directory dgb_instance ( or whatever since the name of the folder doesn't metter )..

I made it under /User/Myusername/dgb

Then I copy the files downloaded from Git repo into /User/Myusername/dgb where I created the .git, databases and drupal folders.
After that I put the entire drupal project files of my web public folder into /User/Myusername/dgb/drupal/

So, into the web public folder ( in my case /Library/WebServer/Documents/myDrupalProject/ ) I run the command drush dgb-backup but seems drush can't find the command.

What I missed?