I didn't have to "activate" any demo, the F@H method worked with every single free demo from the PSN.
There is a small number of links that will need to be initially "activated" or "touched" in order for them to download.
From the Store Links Thread:
Note- there are some demoes that are US exclusive which need to be activated on the US store before they can be played. To do this, log into the US store using a US PSN account and start downloading the actual demo. This will activate the demo on your system. From there, just download the demo via F@H the normal way. IF anyone has any issues with this feel free to post below.
List of games that currently need this method: NHL 08, MX vs ATV, and possibly Def Jam Icon.