Problem is that your 2007 has an older Mini-DVI output port and Apple never made or supported Mini-DVI to HDMI.
So on that note: I can only suggest some alternate solutions that may or may not work for you.
1. Trying some after market > Mini DVI to HDMI adapters - Google Search
2. Exchanging that HDMI only Dell and get a Monitor that also supports DVI, then use the Apple standard Apple Mini-DVI to DVI Adapter - Apple Store (U.S.) and a good DVI cable.
Well @den.thed the sad part is that here in India they don't take products back even if you've bought them a couple of hours back! :( so that can't be done...
But I found something and I would be very happy if you could let me know if it's the right one :
Since they won't take this back after the purchase so it has to be the right one or else all my money will go to waste!