After you get info on the file you want to open such as .jpg, and select open with.
Select CS5 and make sure you have the padlock locked before you select Change all...
Although I don't know why you would want CS5 instead of CS5.5
Thanks for the suggestion, but it behaves the same, even with the padlock unlocked. Click change all and it reverts to 5.5.
As for why I need CS5, we distribute files worldwide and need to meet the compatibility levels of not just our clients, but their printers.
As such, we migrate to new versions relatively slowly. That said, 5.5 doesn't really bring much to the party for us anyway. It'll be at least 6 months before we evaluate CS6 for production, even though we've already got it in-house.
Just to make sure, select cmd I for the file type you want to open in CS5. Scroll down and make sure padlock is locked. Under Open with: select Adobe Photoshop CS5. Then click on Change All....
You definitely need the padlock locked when you do it or it will go back.
If you aren't using CS5.5 just uninstall it and don't de-activate which will allow you to reinstall later.
It may not be the solution you want but how about putting 5 and 5.5 on separate user names/accounts?