In general, I've found the Nik plug-ins to be the least problematic of all that I use. Unfortunately you haven't provided enough information so that help can be focused on your problem. So here are some of the standard steps to take.
1. Make sure you are running Nik's latest versions. There were earlier ones that didn't work properly with ML.
2. Which plug-ings are the problem?
3. Have you tried the repair/rebuild option on your Aperture database? Often what appears to be problems with a 3rd party item actually stem from some sort of fault in the database.
4. If you're really stuck do the following steps: A. Delete the Nik plug-ings. B. Repair/rebuild the database. C. Download Nik's latest versions. D. Reinstall. E. Test.
This is a generic problem solving approach that fixes many things. Also, you should examine if it is a particularly type of file or set of photos or something specific in the images you are working on. A corrupt image, for example, can produce really weird things on occasion.
Hopefully some of this is useful. If not, more detail is needed.
As for the hang during export, is it always the same photos that Aperture hangs on? Does it happen in other libraries? Have you tried exporting photos to a different drive?
Oop, I see that SWSonora's response answered the question. Leaving this here in case anybody else asks the same question.
Message was edited by: Aye Es Oh