Probably it can't merge in libraries in the older format.
I'd suggest opening the older library in Aperture 3.4.2 and letting it update it. Then once you've done that, re-open your "main" library and try re-importing the old library.
Do be aware once the library has been "updated" that your old machine on Snow Leopard (that's 10.6.8 BTW; 10.5.8 would be "Leopard") won't be able to open it. If you need to be able to do that you should duplicate it before upgrading it.