WordPress on OS X Lion 10.7 and Mountain Lion 10.8 error

Hi experts,
When I want to install WordPress on my MAC with X Lion OS this error appears. I don’t know what I cannot install it because is downloaded from a trusted website and is a trusted source. Any solution ? Why I have this and what should I do to make the install properly ?
Thank you experts !

“bitnami-wordpress-3.4-0-osx-x86-installer” comes from an unidentified developer.
Your security preferences are set to block installation of applications from unidentified developers.
Safari downloaded this file today at 3:32 PM from bitnami.org.
