Phil;
Both are dead easy to get to via the RM23 motorway which runs past C de A :
Alhama de Murcia is about 15 mins away. From Condado, you need to turn right and go up the RM23 until it ends at the rounadabout (about 5km), then turn left on the roundabout - signposted RM3 to Murcia/ Alhama De Murcia. Stay on that road for about 5 KM and it will end - you are then in Alhama. The town centre is well signposted and easy to get to.
Mazaron is in the oppostie direction - so it's left down the RM23. After about 10 KM you will pass Camposol - keep going for another 2 or 3 juntions (another 10 KM or so) and you'll see Mazazorn is well signposted. It's no more that 2km off the motorway...
If you want to go to Puerto De Mazaron, (beaches, Marinas etc) you need to go past the Mazaron exit on the RM23 and continue to the end of the Motorway. At the rounadabout where it ends, turn left and then follow the signs over the next rouandabout to Bolneuevo / Pto De Mazaron. Follow the raud 'round the side of the hill and you'll come to another rounadabout with the sea in front of you. Turn right to go to Bolneuevo (beaches, couple of restaurants) or left to go into Pto De Mazaron (another 2-3 km futher on). You'll find a selection of beaches, cafes and restaruants on your right as you drive. It's about 20 - 25 mins drive to Pto De Mazaron, depending on traffic and the engine in your hire car(!) - but 10 mins of that is getting out of Condado..
Cheers
Russ