I use the domino-import wizards a lot..
Noticed this behaviour whilst using the 4.5.10-wizard against Zimbra5.0.1. (testing this because of conversion bug which leaves subjects on imported messages blank ()
So: are you using the same import-wizard version as zimbra version?
Second: on resolving hostname: both zimbra-server and domino-server can be addressed by just their ip-number, you should try that if your dns-names don't resolve well.
Third: make sure no firewalls are blocking traffic from your domino-server to your zimbra-server.
On which machine are you actually running the wizard? It needs to have the notes-client installed so it might be easy/simple to test on a different machine, starting out on your actual domino-server.
Hope some of this helps.
Using ZCS Network-edition 5.0.16 on Ubuntu 6.06.2 LTS and 8.04 LTS