To cite with a specific page, or a page range, do the following:
- Right-click the citation, then choose “Edit Citation”.
- Enter the page number or page range you wish to cite.
The ability to cite page ranges is made available in the 1.0.5 update, presently available as a beta version. You can update to the most recent version by going to Manuscripts > Check for Updates… or by downloading the most recent version from here: http://updates.manuscriptsapp.com/apps/manuscripts/download (then replace the version you have put in /Applications with the one you downloaded).