First, yep, @jerrytron is correct, Spark Core can remember up to 7 sets of network credentials.
Second, here's how you can get the Spark Core's MAC address.
Go to https://www.spark.io/build and paste the code from this gist:
Build and flash it to your Core either over the air if it's online or using dfu-util.
The Core will send its MAC address 3 different ways, you can choose the one that's easiest for you:
- Over USB serial every 10 seconds (screen, putty, coolterm, minicom, etc.)
- Published (privately) to a Spark event stream every 10 seconds
spark subscribe mine with the Spark command line tool)
- Variable readable via the API
When you're all finished follow the instructions in the Getting Started category to put Choosatron firmware back on the Spark Core!