Thanks, Cameron
I concur that the Carrefour people (in my case - the Prat branch) had no idea on how to install the SIM in my iPad or how to config it (ie - enter the APN settings).
But in my case they were very fast and helpful while purchasing and activating the SIM.
Since I had an iPad, I had to cut to size the regular SIM to microsim size, using a cutter I had previously purchased on eBay. The actual cutting process was straightforward.
Since I don't switch SIM cards on my iPad, I'm embarrassed to say it took me a couple of minutes to remember where the slot was, this was the greatest delay in the process.
After that I quickly found the APN in the manual provided with the card.
I was not able to browse, so I asked the sales person to verify that the SIM was indeed activated (and it was, I know this because I saw the 3G icon).
I then rechecked that I had properly entered the APN, and indeed, it was incorrectl (lowercase rather than uppercase). No user/password were required.
After correcting the APN, I was able to browse using 3G and had no further issues throughout the trip.
The only thing (apparently unrelated to Carrefour) was, as I wrote, that deep inside the various nature reserves we had no cellular coverage, neither with the iPad or with our own cellular phones (non-Spanish SIM cards).
The sales person was helpful in pointing out that if I paid only one more Euro, I would have a full month usage.
I do agree with Cameron that it is better to go to a specialist store, where they do this stuff for a living.
In my case it was not an option since we did not go straight to Barcelona from the airport, nor was there any The Phone House branch along our 1st day route.
And in our case our flight was 5 hours late, so this basically was the only viable option I had.