The out of date message really has no meaning other than a warning from Blizzard that a given AddOn *may* be incompatible. Blizzard, each time you have to accept the Terms of Service, disables outdated AddOns.
The solution is simple:
- On the character select screen, click the AddOns button on the lower left
- In the new window, top right, turn on load out of date addons
- Accept the change, and load the game
If, after that, an AddOn still does not work, then it truly is not compatible, not just possibly incompatible. Many AddOn authors never update the compatibility flag in their AddOns if the AddOns actually still work.
Think of it this way: Microsoft Office 2010 still works on Windows 8; making Office 2013, assuming no other change than changing the year, is not necessary. However, if Office 2010 did not work on Windows 8, then Microsoft releases Offfice 2013 that makes all required changes.
Anyway, back to AddOns: remember to turn that flag on every time you are asked to accept the Terms of Service.