Hmm... could sound like you copied and pasted everything I wrote, and not just added the US-language reference parts...? *ponder*
If that is what you did, you need to have the game's installation part in the target box there unmodified and only add
/env:usmain /lang:en_USto what is already there.
So, say for example you installed the game under
C:\Programme\Sony\Everquest2 then the target box should say already say something like
C:\Programme\Sony\Everquest2\EQ2.exeand when you add the language reference it should say
C:\Programme\Sony\Everquest2\EQ2.exe /env:usmain /lang:en_USIf your game installation path is different from the example you would have to either leave what is already in the target box unchanged, or change it to reflect the directory name where the game is actually installed...
If you already did this and then got the error message, then it's kind of hard to guess without knowing what you actually changed and what it (the target field box) currently says