Good news: getting employers is relatively easy. Go to other industry job boards (Dice, StackOverflow, etc.), and browse the jobs. Email all of the companies you find ("Hi, I saw that you posted on XYZ, and wanted to let you know about us... we're a free site... easy to use... 5 mintues... etc"). If the cost is low (time + money), they will give it a shot.
Or, if you're feeling bold and a little scummy, just simply scrape the content from the other sites like SimplyHired or Indeed. The other sites won't like it, but since employers are really the only ones in a position to copmlain/whine about that, you should be fine; why would they care if you're just giving their ad more views?
Bad news: getting job seekers is nearly impossible. Especially in the developer/IT niche. It doesn't matter if you have a hundred thousand awesome and unique jobs, and there's a damn good chance that anyone who visits the site will find their dream job. You still need to build your brand and pursaude qualified job seekers to come.
My company, Inedo Media is a niche advertising agency that exclusively works with companies in this space, and I can tell you first hand... it will be expensive to launch this and get any traction. Think tens of thousands a month over six to twelve months, and maybe -- just maybe -- you'll have a shot.
Oh, and that's assuming you're working with an experienced team of marketers like us. And finding that team will be a challenge; truth be told, you'd have a hard time convincing us to take on such a long-shot project. Do it on your own, and you may as well just have a cash bonfire.