Actually it's not available on the Xbox Live Arcade, and it doesn't fit on a memory card so it can't be transferred that way (and even if it did fit, which it doesn't, it could only be transferred to a second Xbox360 as a trial-version).
Having said that, all is not lost. If you have registered your console on Xbox.com, you can call customer services and tell them your problem and they'll send you a code. You type the code into your Xbox360 and you can then download the full version for free.
A quick warning, though: it's not a quick process. When you call the support people they won't give you the code straight away -- they'll take your details and tell you they'll call back with the code. It took eight days and many phone calls for me to get mine...
However, if you're prepared to jump through the MS hoops, you can get Hexic back onto your console.
Hope this helps.
Steve.