I just bought a fast little PC laptop and I want to put a second copy of Sonar Platinum on it. I know I'm entitled to a second copy, but not sure how to get it. Someone please point to the right place.

You don't need a second copy. Just install the copy you have using the same method you did before (on your original machine) and call it a day.
 
You are allowed to install on multiple machines as long as...
 
a) You are the sole (main) user
 
b) Only use one "copy" at a time (which is more a technicality just to prevent you from letting a bunch of other people use your copy)
 
If you TRULY wanted to PURCHASE an extra copy (with a new s/n) then I think you'd just set up another CakeWalk account specifically for that new copy... but they may have mechanism for multi license purchases so that would be a Customer Support query.
 
Edit: The gist, there is nothing in the User Agreement nor is there any programmed in impedements to installing the same copy of Sonar on multiple machines. The Bakers have also said they do not care about said practice as long as it's ONE owner/user installing it on THEIR systems for THEIR personal use.
 
Sumthin' like that... so don't worry about it.

Use the Command Center or the installers you have already downloaded. Only Melodyne and XLN actually restrict you to two installations. Cakewalk does not restrict the number of installations; you are expected to use one instance at a time.

So If I'm not using ANY of my copies of SONAR, then I'm really not a licensing issue, ya?