Hun in the Sun Campaign problems

[huninthesun]

There's no harm in trying to install again. You said you can see the entry for the campaigns in your Single Player menu - what does happen when you click on the menu? Does it change to another screen or is there no response at all?
Check the files and folders under the Steam\steamapps\common\il-2 sturmovik cliffs of dover\parts\huninthesun folder against the those in the installation file. If you have the DLLs and the mission\campaign\campaigns.ini file then selecting the campaign from the menu should take you to another screen that allows you to choose either the Hurri or Spitfire campaign.