My apolagies. Your initial question didn't indicate that you had tried anything else. The support website seems to be the only information around for this machine and the spec sheet says it comes with 2 x 512 modules. If CPU-Z is telling you there is a single stick of 1mb ram installed then you just need to find the other slot(s). Perhaps the other slot is hidden under your existing module?
Some of the people on this site seem to believe this is actually a Twinhead H12Y.
http://www.itreviews.co.uk/discuss/2409/9
On page 7 of that forum, one person said they installed a 2g memory module and it worked fine.
I coulnd't find the H12Y on the Twinhead site but the models that have the C2D seem to come either with single or dual slots. Maybe you could match your processor to one of the models for the counry of purchase and get a little more information.
Typically, when they say that it will only support X amount of ram, it means they haven't tested it with all possible configurations. As mondoman pointed out, you should be able to find out what modules will work from different manufacturers. But that's dependant on how many slots are physically present.
You seem to have a very perplexing problem with limited tech support.
edit: Just found this:
RAM is under the metal panel on base of the machine (Slot 0 is the top of the two slots so you'll need to remove the existing DIMM to fit the new one i slot 1 and then refit the original one back in 0)
It's at the bottom of page 16 of the forum I pointed to earlier. Hopefully it's the same model and configuration as yours.