On character selection screen, at the bottom left you can see "AddOns" click that, enable desired AddOns, and tick that Load Out of Date -box.
There are no AddOns on that list? Go to your WoW/Interface/AddOns/ -folder, and check if the AddOns are packed within an extra folder. This is usually the issue when AddOns aren't showing up, despite being for the correct WoW version, and installed within the right folder.
E.g. the addon might be installed like this:
WoW/Interface/AddOns/DBM/DBM
WoW/Interface/AddOns/DBM/DBM-TBC
When the addon should be installed like this:
WoW/Interface/AddOns/DBM
WoW/Interface/AddOns/DBM-TBC
Example image of WoW/Interface/AddOns -folder.