Apparently TBird has discontinued the Eudora import function, that's why it's grayed out. I had to do this a few weeks ago, and there is a guy who wrote (for himself and posted for the public) a converter (but it runs only on PowerPC Macs, or running a PPC emulator on Intel chips -- and I was lucky to have an old MacBook running Snow Leopard that still had the Rosetta PPC emulator -- you might be able to use VMware Fusion too, I know of someone using that on a recent Mac in order to continue using Eudora, and the utility should work on that if Eudora can work on that). Search the web for "Eudora Mailbox Cleaner" if you can use the required OS. I think there may be a different utility somewhere for Windows, but it was not relevant to my needs.

just install an older version of Thunderbird like 17 and do your import than reinstall the new version. Simple and effective unless you have a weird Eudora setup

http://ftp.mozilla.org/pub/thunderbird/releases/17.0.9esr/win32/en-US/

OK, I have installed version 17, and I do get further: I can now select import from Eudora. However, it fails to 'find' any of my Eudora data!! It exists in the usual place in my user appData folder. I have numerous mailboxes, a substantial address book, and many filters. None of which are found and imported.

is Eudora installed and able to run? My guess is no, hence the problem that the relevant registry keys used by Eudora to point to the Eudora data location are not set.

See http://www.tenforums.com/browsers-email/45272-eudora-7-fails-load-win10.html

See also http://bahut.alma.ch/2007/09/importing-eudora-address-books-into_10.html and http://kb.mozillazine.org/Importing_from_Eudora_(Thunderbird)

I am able to run Eudora. Just to be sure I tried it just now.