When does Myr Superion check the source?
The ability checks what produced the mana. This means that, to be used to cast Myr Superion, the source of the ability that produced the mana must have been a creature at the time the mana was produced (i.e. when the ability resolved).

As @Natedogg2 of #magicjudges-rules said, "Producing the mana, then making the source into a creature does not count."