There are a few reasons why Planapple might not accept a forwarded email for your trip:
Emails are always added to your “current” trip — the last trip you were viewing on Planapple before forwarding the message. Go to “My Trips”, click on your current trip and make sure the email isn’t there (in case you were expecting it to be in a different trip).
You need to forward the email from an email address that is “verified” — meaning that you got a confirmation email and clicked through to verify that you own that email address. Sometimes these confirmation emails can end up lost or in spam folders, but you can send another one. Go to your email settings and click “resend verification email” to send the confirmation email again.
The "from" address on the message you forward must exactly match an email address you've registered with your Planapple account. If you're forwarding from a Gmail address, double check this: Gmail will let you use either "email@example.com" or "firstname.lastname@example.org", but Planapple sees those as different email addresses. If you're using both as "from" addresses, you'll need to add both emails to your Planapple account.
If your "from" address is registered to more than one Planapple account, Planapple will reject any email forwarded from it. If you think you might have multiple Planapple accounts, you can double-check by sending yourself a password-reset email: you'll get multiple emails if you have duplicate accounts. (You don't need to actually change your password.)
You can add (or remove) email addresses to your Planapple account in your email settings. Email from any of your verified addresses will show up in your current trip.