i had that error 5 hours ago and it was pissing me off really bad
first of all
make sure you entered the key correctly
that can be a problem
next, make sure that the directory that opens d2loader when you're starting the bot is valid and exists in c:\program files\diablo ii\diablo ii loader.exe
if it still messed up, it might be because of the new .dll patch
i'd give you the link for the patch, but it's from different forums + you have to register before you can dl it. am i allowed to do that? haha