That's right -- Screen Import will automatically detect the "* Exported from MasterCook *" style format and import it. Pretty cool, no?
If your version supports export to MX2 format, then BigOven should be able to read that. If your version supports export to MXP format, BigOven should be able to read that as well. But some very, very early versions of MasterCook (8+ years old) use a slightly different (older) syntax, which isn't supported.