the D10 supports both SAS RAID via an LSI 1068E controller and SATA RAID supported by both the Intel ICH9R controller and the LSI controller. the only downside is that both controllers cannot be used for RAID simultaneously.
if you already have one 146.8 GB 15K.5 hooked up to the LSI controller then you'll need to source a second SAS cable. the FRU number you need is 41R5513 (which should also be found on a white tag on the existing cable in your system).
to enable SAS RAID, it needs to be enabled in the BIOS. this is found under the Advanced tab, then under LSI SAS Interface. if it's already enabled then proceed.
if you want to do a non-destructive RAID 1 mirror (known in LSI speak as "IM" or "integrated mirrored") then you can do this through the controller BIOS by pressing ctrl+C at POST to enter the LSI BIOS. you'll probably want to read the setup instructions in the D10 HMM as it's much easier than retyping everything. links to all manuals and guides can be found in this thread. at the bottom you'll also find a resource in buying parts.
alternatively, you can download the latest version of LSI MegaRAID Storage Manager from LSI's site and use a GUI. for new users this is a little easier.
if you want to use RAID 0 (or "IS" / integrated striped) then this is a destructive build and will have to be done from the LSI BIOS. be sure to back up all data as everything on the 15K.5 will be wiped in the process.
read and follow the HMM. if you have any questions on the process or need further parts info, please don't hesitate to ask.