Create an USA account, login into that account and install these demos (Narnia, Wall-e and Monster Madness) using the method explained here:
I normally use the method above, and the sithvegeta's method only to install demos that are not available in the PSN store.
aldostools, thanks! I had to download it again via the PSN, but then it installed.
The proxy method did not work, even with the USA account logged in. But at least I can download the new demos from the PSN directly now, and I can install them.