The options discussed on this page are found by selecting Menu --> Setup --> System --> PC --> PC Settings -->

FLEX does not integrate with a Phoropter Head directly, but the Thomson Software which accompanies it. You can Pull / Push RX's to and from the phoropter head within the Clinical Refraction pages of the Sight Exam. The Topcon CV5000 will link, as will the Nidek RT5100.

If you use the Thomson Software phoropter head software, you can link this in under Menu - Setup - System - PC. Choose the model as shown and set the 'Phoropter Path' as the location of said software. Optinet or Thomson should be able to assist with this.
Next, under Menu - Setup - Examinations - Clinical - Refraction, you can then choose which 'Phoropter Rx' your refraction tab will link to.  
Finally, in the Sight Exam, you can press 'External Link' and choose 'Thomson Software'. Here, you can 'pull' (import) and 'push' (export@ Rx's to and from the phoropter head.