This occurs for one of two reasons:
1. If you've skipped some or all of the permissions requested by Facebook. All permissions must be accepted in order for Facebook to provide us with the pages you manage. To address this, please see this page for steps to revoke the Sendible App on Facebook so that permissions can be re-accepted.
2. The Facebook Profile you're logged into has not been given Admin access for the Page/s you're looking to add via Facebook Manager. In order to accept App permissions, you must have Administrator access to the Facebook Page/s you're looking to add. Please see this page for more information on Facebook Page roles. Once you have been given Admin access or have logged into the Facebook Profile that is Admin of the page, the relevant Facebook Pages should then appear in the dropdown menu.