As you are trying to add another PC I will assume that you are an established user and that you have set up your Account online. If that is the case then log into your Account Console. Once logged in, click on your email address (in the upper right hand corner of the screen) and then select "Manage Keycodes" from the resulting dropdown menu.
If for some reason you are not able to log into the Account Console, then please follow This Link to get hold of the download instructions and have the key code re-emailed to you. :D
Hope that sorts you out? Do post back to let us know how you get on/if you have any further issues, and have a great weekend.