To avoid having to continually select the Add-in from the main menu, simply pin it.

TIP: If you want the Add-in to be present both when reading incoming messages and when composing emails, be sure to pin it in both windows

If the Add-in is still disappearing when you select another message, the most likely reason is that you have 2  or more email accounts hosted in Outlook and the message you have selected is not in your business email account; the Add-in will only appear for emails that are in your business email server, as that is where CloudFiler is enabled. For example if you have a personal Gmail account hosted in Outlook, when you select your Gmail emails, CloudFiler will disappear as it can only file email on your business email server.