Can you show to me a screenshot of your custom parameter configuration, just like I did in my previous comment?
For this work user.addCParam(MSXP,1) you need to set the shortName in quotes:
- user.addCParam("MSXP",1);
Besides that, the $gameParty.members()[0].MSXP should work. Which lead me to think that maybe it is a compatibility issue.
Can you provide me a sample project replicating the error so I can take a look?