Comment 27 for bug 445487

There is a further easy workaround which works also with nm-connection-editor on karmic. Just concatenate your privatekey together with your cert like

  cat key.pem cert.pem > mynewkey.pem # key.pem needs to be encryted with a password!

Then use mynewkey.pem as new "private key" and the Apply button should appear
(thx to https://bugzilla.gnome.org/show_bug.cgi?id=585570#c6)

But it looks like the bug is already fixed in upstream - I have no idea if this patch is already included in Lucid or not (?):

git log NetworkManager/libnm-util/nm-setting-8021x.c
commit 4274edf47dfc1341704311d9587dc6951db480e9
Author: Dan Williams <email address hidden>
Date: Tue Dec 8 16:52:24 2009 -0800

    libnm-util: client certificate should not be required to set private key (bgo #585570)

    Not sure what I was thinking originally; there's no way this was
    correct in the first place.