Depending on the profile associated with a Salesforce user, there's a chance that your user might not see Soapbox Mailer in the app menu in the upper-right corner of their screen in Salesforce. Typically, this is because the profile they're using hasn't been given permission to access the app.
Here's how to provide a user access to Soapbox Mailer by modifying their profile in Salesforce.
1) Determine which profile the user is using in Salesforce by going to Setup > Administration Setup > Manager Users > Users: and clicking on the Full Name link for the user in question
2) On the User Detail page, note the Profile that this user is associated with (found in the upper-right side of this page's list of options)
3) Go to Setup > Administration Setup > Manager Users > Profiles to view all the profiles you have setup in Salesforce
4) On the Profiles overview page (listing all the profiles, click name of the Profile of the user in question
5) On the Profile page for the profile in question, under the Apps section, click the link titled "Assigned Apps"
6) On the Assigned Apps page for the profile in question, click the Edit button
7) In the edit mode for the Assigned Apps page, in the Soapbox Mailer row, check the box in the Visible column and then click the Save button
That's it! All users using this profile will now have access to the Soapbox Mailer app.