Please see this link for the download : Download CS5 products
and see these instructions to extract the files and install your product.
You will need the serial number you got when you purchased it in order to activate the software.
You can install and activate (purchased) perpetual software two computers of the same type (mac or pc), as long as you only use it on one at a time. (lightroom multi platform product is the exception as it can be installed on both pc and mac platforms)
It is a good practice to deactivate software on a computer where it will no longer be used, especially before giving away or selling the computer, or reformatting the hard disk. Please see this link: how to deactivate software. This frees up one activation so you have the option to install the software on another computer at any time.
Pattie