Welcome to Apple Support Communities
First, start the MacBook holding Command and R keys to start into OS X Recovery, and open Disk Utility.
Then, choose the second "Macintosh HD" option you see in the sidebar and verify the disk. If it's damaged, repair the drive. Finally, restart and see if the hard drive is OK
Alright here is the deal, i tried what you advised me to.
When i chose the second "Macintosh HD" option in the sidebar, the verify disk or repair disk options we not highlighted. I could not click on them. I have attached a snapshot of this. Also i went back to the first "Macintosh HD and verified it and repaired it as well, nothing happened, still the space is not free. Please take a look at the screenshots.
Make a backup of your drive with Time Machine > http://support.apple.com/kb/HT1427
Then, hold Command, Option (Alt) and R keys while your Mac is starting, and open Disk Utility again. Choose the first "Macintosh HD" option in the sidebar, go to Partition bar and, when you see "Current", press "1 partition". Finally, apply the changes.
After that, close Disk Utility, select the option to restore a Time Machine backup and follow its steps.
Something is damaged in your drive and has to be repaired erasing the drive