The process for obtaining the Call of Duty MW3 Collection 1 DLC is slightly different from normal PS Store purchases.
To make the process as smooth as possible I have detailed the steps below.
Enter the Modern Warfare 3 category on the PS Store via the in-store.
Purchase the product called Call of Duty®: Modern Warfare® 3 – Collection 1 and you will see the Purchase Completion screen displaying the message Thank you. Your purchase has been completed. At this point you will not have to download anything.
Select the "continue shopping" option then "back", this will return you to the Call of Duty Category.
There will be 7 different products, one for each disc version (e.g. BLES01429, BLES01433, etc).
Choose the version that matches the BLES code on your game disk (this code can also be found on the spine of your game box) and the 6 maps and access key will be added to your download list.
Download and install!
If you select the wrong version then don't panic, you return to the Modern Warfare 3 category at any time and select the correct version.
Any questions please ask.