A quick way to add reminders to the Reminders app is to drag in an email message directly from your Inbox. The message is then converted to a reminder task, which you can then edit, if you so choose. The task also includes a “Show in Mail” link that will display the original message in the Mail app.
There is one caveat however – there must be already at least one existing reminder in your reminders list in order for this trick to work. You will be unable to drag in an email to an empty reminders screen.