Why does the Resubscribe link lead to a 404 Page Not Found error?

Problem: 

When clicking on the unsubscribe link in a campaign you receive a simple and great unsubscribe page and notice.

Unfortunately when clicking Re-Subscribe you encounter a 404 Error.

What do I need to do to correct this error?

Solution:

You likely haven't upgraded to the latest version of the mailer plugin.  You can do that by:

1) logging into your SFDC instance

2) go to this URL: https://login.salesforce.com/packaging/installPackage.apexp?p0=04td0000000AqtW

3) following the onscreen instructions from there to finish installing the package.