Why is "'Email_Links__r' is not a valid child relationship name for entity Campaign" appearing on Edit HTML Body pages?

When creating or editing a Soapbox Mailer campaign, a user might run into the following error page after clicking the Campaign buttons "Edit HTML Body" or "Edit Text Body":

"'Email_Links__r' is not a valid child relationship name for entity Campaign"

This error is caused because the current user's profile does not have read access to the Email Links object that is created when you install Soapbox Mailer.  To correct this, you need to provide this user's profile access to at least "read" privileges (preferably "edit" privileges if this profile should be allowed to create Email Links for emails) as follows.

1) Go to Setup > Administration Setup > Manage Users > Profiles

2) Click on the name of the profile of the logged in user

3) On the logged in user's associated profile page, in the Apps section, click the Object Settings link

4) On the Object Settings page, click the link for Email Links

5) On the Email Links page, click the Edit button

6) In the edit mode for the Email Links, in the Object permissions section, check the boxes in the Enabled column for the following Permission Names:  Read, Create, Edit, Delete

7) Click the Save button

Now, the user profile in question will have Read, Create, Edit, and Delete access for creating Email Links.

Happy emailing!