First, backup your library.
Then, try repairing your library. Try the import again, if that doesn't work, try rebuilding it.
If that doesn't work, export a few projects as new libraries. Try that again. Some have had success in breaking up their libraries until they find the offending images/metadata.
If you have aperture albums that reference images in several projects, you may get some duplicate images--not sure, just speculation on my part. So you might want to logically group projects.
Thanks for the suggestions.
To date I have:
repaired the Aperture library
rebuilt the Aperture library
exported 3 separate small projects as libraries (Including one with only jpg files)
At each stage I have attempted an import using the plugin and got the same message on every occasion
I have now deleted the plugin and downloaded the latest plugin. I am now getting the following message
An internal error has occurred.
[string "Import.lua"]:157: attempt to index local 'kwdInf' (a nil value)
Sorry, did not receive an update from the forum about your posts, and that the repair / segmentation effort did not work. I'm not sure what to tell you here. The messages are cryptic because Adobe is using scripting within the plugin for the migration.
Sounds like you really need to speak with support. You can try that next, but I've never had success with Adobe support. They'll help you install Lightroom, and that's about it.
One last thing you could try: Create a new Aperture library and import new images. Export master images from another library if you must. Does this work? If not, you may have an underlying issue such as permissions (try repairing permissions in disk utility), or application library corruption. Another step could be to create a new user and try this new library (or old libraries) with the plug-in. Just make sure the new user has permission to access these libraries. An external drive with permissions turned off is a good way to go.
Thanks very much for your help.
The only option that worked was to export original files, Import them into a clean Aperture database and then run the Lightroom plug in.
This has saved me many hours of work.