To do a dual drive setup you'll need a special carrier to hold the HD to then be placed into the location the optical drive was. Then the original optical drive cable plugs into the carrier.
If you haven't bought your SSD yet (or can return it). How about doing something better, we gave up doing dual drive setups as often you'll encounter issues of performance & reliability. You see the SATA port your HD is currently in has a sensor system built in (the optical drive SATA port doesn't) to help protect the HD from damage (crash guard). The optical drive port is also slower than the HD port. In some models the port is so slow you can't migrate the HD over.
OK, heres the better way:
Seagate offers a SSHD unit which is a hybrid drive having both a SSD for a deep cache giving you the speed and your traditional HD for its deep storage. Often people are also looking at increasing the HD size as well so this is a good time to do this as well.
Heres the drive info: Seagate Laptop SSHD and heres the spec sheet: Laptop SSHD.
In any case make sure the SSD you get is able to run as SATA II (3.0 Gb/s) and you'll also want to make sure your EFI firmware is up to date follow this Apple TN: EFI and SMC firmware updates for Intel-based Macs.