I'm afraid what あなた want is not possible on Fanpop. あなた can only receive Eメール notifications on フォーラ topics that あなた started yourself.
The only thing I could recommend is saving the フォーラ あなた would like アップデート on as a お気に入り (little green button in the 上, ページのトップへ right corner of the フォーラ topic). If あなた do that, the link to the フォーラ topic will be on your プロフィール page (left center), so あなた can easily check for new 返信 every time あなた visit Fanpop.
I know this isn't exactly what あなた were looking for, but it's the 次 best thing.
