Good question. Here is my take on this.
Adobe is not selling Premiere Elements 11 anymore. It is now selling its current version Premiere Elements 12. And, its free tryout download from Adobe is for Premiere Elements 12. And, no problem purchasing a serial number for version 12 from them.
There should be no problem in trying out the Premiere Elements 11 as per the installation disc that came with the Photoshop Elements 11 purchase. But, when it comes to purchase of Premiere Elements 11, the only scenario that I am seeing is
Find an authorized reseller who still has Premiere Elements 11 stock. Purchase that product. And, then you have 2 choices
a. insert the serial number that comes with the purchased Premiere Elements 11 into the tryout already installed on your computer
b. uninstall the tryout, install your purchased Premiere Elements 11 with the serial number that comes with it.
A typical place that might have a Premiere Elements 11 in stock is Amazon. Have you looked there?
You could contact Adobe for input on the matter, but I suspect that you will get steered to purchasing Photoshop Elements 12 and Premiere Elements 12.
Adobe chat MAY be able to help... but I think ATR is correct about not being able to buy a version 11 number from Adobe
Select a reason, then click I STILL NEED HELP to activate Premiere Elements chat
As I stated in my post
You could contact Adobe for input on the matter, but I suspect that you will get steered to purchasing Photoshop Elements 12 and Premiere Elements 12
Subsequently John has posted encouraging to see what Adobe has to say about this matter. Worth a try. But I am 99.9999% sure that Adobe will try to sell you Premiere Elements 12 or more. I just want to have you prepared in advance one way or the other for the response that you could get from Adobe.
If it comes of a purchase of Premiere Elements 12, then consider getting Photoshop Elements 12 and Premiere Elements 12 to leave open integration perks between the two that would not be available under a Photoshop Elements 11 and Premiere Elements 12 setup.
Before you call Adobe, look at the price for what is available at Amazon for Premiere Elements 11 and compare that price with the price of Photoshop Elements 12 Premiere Elements 12 bundle from Adobe or elsewhere.
We will be watching for further developments.