I'm using the short name of MSXP and I've written the same way in the numerous places, but couldn't get it to work that way at all. For instance, $gameParty.members()[0].MSXP wouldn't return the value, but $gameParty.members()[0].cparam(3) would. I also used a.MSXP + a.atk * 4 - b.def * 2 in my damage formula but I get the same amount of damage every time. I had this in the note tag for a skill and it didn't work:
<JS Post-Apply>
user.addCParam(MSXP,1);
</JS Post-Apply>
But using 3 instead works.