Get a business presentation together and set up a meeting. If you take it seriously they'll probably take you seriously. Another route would be to get a rough working copy of the game together and spam it out, drum up some underground interest.

I'm in the same boat as you are.

I basically did a search for game developers. Contacted them to find out what they needed for a PITCH. Most companies want a storyboard format of the main story or quest, and then other options as far as if it is a open format like an rpg or if its just a straight forward move through levels kind of game. They are looking for time periods - locations that the game takes place in or around - character development - marketability - and public appeal. If you think you can just walk in and tell them your idea, you won't be taken seriously. They want to know you've done some research on your idea. Most importantly - can your idea be made into an original game, or are you violating copyright laws.