Are you getting an error message? If so what does it say?
If you are not getting an error message go to Preferences > Genera tab > at the lower end of the dialog box, Prompts click on "Reset all warning dialogs." Tray to use the "edit in function" do you now get an error message?
Would suggest you to try the suggestions mentioned here. Lightroom: Edit In External Editor does not work/function
Also, make sure your Lightroom and Photoshop both are up-to-date.