Unfortunately, plain RAID 0 is not a feature of this NAS, as the previous poster explained. If the shop promised you RAID 0, then you should return it as it was falsely sold. There is a different version of the product available by some companies (e.g. Fujitzu Siemens) with Windows Home Server, which MAY allow you to buld a RAID 0 array. You may be able to axchange it for that, though it is likely to be tricky as few merchants carry both products, especially in Greece where the original poster is (like me).
I have to be honest though and tell you, that the idea of storing 3.2 TB of data, on a RAID 0 array of four disks, on a system like this, strikes me as extremely high risk:
First of all, it's unnecessary. Any PC with an intel motherboard and an ICH9 or ICH10 controller can do the same, with no need for a specialized box with specialized software. This NAS is geared at people who need an easy RAID5 solution, providing space-efficient redundant data storage.
Second, RAID 0 is generally for high speed, low risk storage eg, running an application or operating system, not for data storage. For that you need safe, moderate speed storage, e.g. RAID5 (or 1). The chance of 1 in 4 disks eventually failing and destroying all your 3.2 TB of data are huge. You are definitely looking for trouble.
Third, this NAS will give you excellent performance under RAID 5 and protect your data at the same time, with up to 1 disk redundancy, i.e. you can have any 1 disk fail and still recover your data by replacing it with a new one.
In this configuration it will give you 2.9TB of storage with 1TB disks, or you can experiment with the new 2TB disks, although they are not officially supported at the moment. If you do, make sure you do not use Green Power or similar Low Energy disks as these are not suitable for NAS. But if you load it up with 4 Caviar Black disks, it will give you 5.8TB of storage, which you will likely struggle to fill. Please note I have not done this, and cannot advise you to do it, but another member of this forum has had some succes with it.
I hope this helps!