Have you upgraded to Aperture 3.4.x?
Aperture is now sand-boxed and will only work with sand-boxed, 64-bit plug-ins; see:
Check, if you have the latest versions of your plug-ins, or if you can upgrade them to be compatible with Mt. Lion.
and when I go to use Nik plug - ins it says the developer is not a recognised provider.
Also, try, if you can run your plug-ins, if you disable the security check of the developer in the System Preferences "Security and Privacy" tab:
You will have to click the padlock and enter your admin password to make changes.