I think you need something to make it more obvious that you click there — to me it looks like stylised text on an image. The way people read websites is fairly standard — they expect there to be some kind of bar with links on it and that's how they navigate.
Your website follows this standard well — there's a logo on the top left, a links bar across the top, a nice image in the middle and a contact/address bar across the bottom. Great. People understand this layout, and that makes it easy for them to navigate. Only problem is the nice image isn't an image, it's two links to two different microsites. This breaks that standard layout. Now this isn't a bad thing, but people just won't realise what is is unless you make it very clear that these are links to two microsites.
Never underestimate the stupidity (and the conventional nature) of web users. They expect certain things, and if you don't make it obvious that something else is the case, they just won't click on it. Could something like "Click here to visit the X microsite" work?
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