If you are viewing a Switch Secure Email on your Outlook for Mobile app, clicking on the .switch attachment will not show you the option to open it in the Switch for iOS or Android Client immediately.
To open the secure email, simply tap on the Switch attachment above the email. Once on the attachment's page, click 'Share' which will give you the option to open the Switch Package inside the Switch app.
When selected, you will then be prompted to sign in with your Switch User ID and password if you do not have automatic sign-in enabled on your account.
Your Switch Package will then open inside the Switch mobile app.