Help: Can't download handbrake (video converter) to my MBP

Searched the forum for recommendations of DVD video conversion app to convert video to MP4 for purpose of importing to iTunes library. "Handbrake" came up on tops but my problem is it won't open to install after downloading; like most newly downloaded apps would instead it displays an error msg saying that it can't open handbrake because its from an unidentified delveloper. Anyone with similar experiences or advise to give?? Thanks!