I have tried opening the playerdata file for this mod with NBT explorer, but it cannot read the file. What is the fix for this?

Which playerdata file are you talking about? Is it contained in .minecraft/saves/<world name>/playerdata or in .minecraft/saves/<world name>/playerdata-mca? Because I have no recollection of any statement that says that files in the latter folder can be opened with an "NBT explorer" (whatever that might be).

The latter one. Its a .dat file which usually can be read by NBT Explorer, its is an NBT editor which can read normal player data and the region files for a world.

So nobody told you that this would work for MCA's .dat files as well, right? How do you come to the conclusion that it would? Just because a file has the same extension as another one doesn't mean their formats have to be the same, let alone match the "designated" format at all. You could, for example, change the extension of a .txt file to .mp3. Your media player wouldn't be able to handle that, either.

So then how would you propose that I am to open this file then? Huh?

You don't.