To login to your Mailgun account from your Rackspace cloud control panel, select the "More Products" drop down menu on the left side of the menu bar. Then select "Mailgun" from the options. (see image below)
Choosing Mailgun will take you to another page, where you will have to re-enter your Rackspace password. Once the password has been entered, you will be redirected to your Mailgun control panel.
Note: Access to the Mailgun control panel is only available for the primary contact of your cloud account. (https://mycloud.rackspace.com)
For additional assistance, please submit a support ticket.