Qualcomm has moved on to Eudora 7.1 (still in beta stages) and hence has decided to no longer sell registration codes or provide technical support for earlier versions of it. So if you have a Eudora 6, on your computer and you are getting an error then you can only hope to solve it on your own, a quick guide to the various Edora errors is here: http://www.albnet.net/faq/errorEudo.htm#1
But if you dont want to go through the rough patch of troubleshooting and debugging then here is what you do, migrate to Outlook.
Here is a step by step procedure to import your e-mails and address book from Eudora into Outlook 2007:

• Launch Windows Explorer and go to the folder that holds your Eudora mailboxes.
  To find your Eudora folder on Windows:
  * Search for "eudora.ini" on your computer.
  If you get only one result, the folder containing eudora.ini is your Eudora folder.
  If there are multiple folders containing eudora.ini, you have to open them all to identify "your" Eudora folder. Look for .mbx files and .fol folders that match your setup of mailboxes in Eudora.
  Find Your Eudora Folder Faster on Windows 2000/XP and Later
  If you use Windows 2000, XP or later, try this first:
  * select Run... from the Start menu.
  * Type "%appdata%\Qualcomm\Eudora" (without quotes).
  * Hit Return to go directly to your Eudora folder.
  If the latter does not work, use the method above to locate your Eudora store folder.
• Copy the following files to C:\Program Files\Qualcomm\Eudora folder: (All are required)
  descmap.pce
  NNdbase.txt
  Any file ending in .mbx (In.mbx, Out.mbx, Trash.mbx, etc)
  Any file ending in .toc (In.toc, Out.toc, Trash.toc, etc)
• After copying the files to the Eudora directory in Program Files go ahead and launch Outlook 2007 and if needed configure the application.
• Once Outlook 2007 has launched and the account has been set up, click on “Personal Folders” on the left side of the screen under “Mail Folders”. After selecting the folder, click on File then “Import and Export...”
• Then on the Import and Export Wizard window, select “Import Internet Mail and Addresses” and click Next.
• On the Outlook Import Tool window, make sure “Eudora Pro…” is selected in the top pane and the check boxes in front of “Import Mail” and “Import Address Book” below the pane are checked. Then click Next.
• On the Import Addresses window, select “Replace duplicates with items imported” and click Finish.
• Assuming everything was done correctly, you should now see the Importing Process window. When the process is done, an Import Summary will display. Click OK.

Voila! You have now imported your Eudora emails and address book into Outlook 2007.