If you go to http://www.servicaixa.com/ and search for Queen, I am pretty sure you can select your seats and purchase tickets for the Madrid show.

NOTE: From Queenzone.com member Enricosta:

The instructions to buy tickets by servicaixa.com for the shows in Barcelona and Madrid are writen in catalan. Not the same, of course!
Prices a littles cheaper than in Paris!
I hope those of you that are intending to go to this concert, get good seats...