When you migrated to your new Mac, you are probably now using MacOS X 10.8.3. and Aperture 3.4. Was was your old operating system and Aperture version?
Some plug-ins are no longer compatible, see:
Make sure you have the latest version of Silver-Efex pro, and then run the installer to install it again. If properly installed, the plug-ins should be in your System Library
If installed in either of these folders, Aperture should see the plug-in as an export- or edit plug-in.
JP -- as a registered owner of NIK's plug-in(s), you should have received an email from Google (they purchased NIK) with an upgrade link. If you haven't, you should, just to be in line for whatever news and/or upgrades Google releases. (They recently said that they would continue to develop the NIK Suite.)
The main thing the Google upgrade does is make your currently registered NIK apps into currently registered Google apps. I think it also confirms (and may correct) the installation.
If you intend to continue to use your NIK app(s), I recommend upgrading them to the Google versions.