I am assuming you report that only the left side of your headphones emits sound when plugged into the audio socket of your laptop.
If you're sure you've got the jack pushed ALL the way in, it could be the jack on your headphones or the audio socket on the machine. If your headphones work on another system, then I would take your G74SX-A2 back to the retailer and ask for a new one. Alternatively, if another set of headphones works in your laptop, the Steel Series headphone jack is at fault.
The fact that both of the laptop speakers work correctly indicate the balance for playback is not an issue.
G74SX-A1 - stock hardware - BIOS 202 - 2nd Monitor VISIO VF551XVT