The stores are in most cities and major towns in mainland Spain. There is a map on their website.
Hipermercados Carrefour
It looks like there might not be a store on Menorca - I think the nearest one might be on Mallorca?
The APN settings were not in the SIM pack. Normally these should be stored on the SIM. Instead the SIM had APN settings for Orange - the network that Carrefour is hosted on. These didn't seem to work - so I simply went into the APN menu on my phone and set up an additional APN for Carrefour. Just add an APN with the following settings - NAME=Carrefour, APN=carrefourinternet.
The SIM is probably designed for use in Carrefour dongles - and these probably automatically set up the APN. I was using the SIM in my phone, so I had to set up the APN manually.
I didn't pay too much attention to the speeds - in general they were good in the cities and towns, and sometimes slower in very rural areas. I was mainly web-browsing, emailing, streaming Google maps and listening to streamed radio sometimes - all of these were fine in all 5 places where I was staying. I also used VOIP and it was mostly ok - sometimes a little choppy - but this is normal when you do VOIP over 3G. Carrefour use Orange's network - so the coverage and speeds are probably the same as Orange.
100MBytes per day is the cap on downloading. I never went above 40MBytes any day - so 100MBytes was fine for me. Every day that you use the Internet, €1 will be deducted from your credit. The minimum top-up seems to be €5, although they asked me to put in €10 when I bought the SIM. Hence the cost works out at €1 per day, but the minimum that you can initially buy is 10 days.
Let me know if you have any other questions.