Thunderbird 3 has a nasty habit of trying to get its users to set up IMAP instead of POP3 accounts. It also has an addiction to trying to guess mail settings based on the email address - and getting it badly wrong. It is not a problem if you upgrade from Thunderbird 2 because it just transfers the accounts across, but fresh installs can be really tricky.
Here's the way round it.
You need to input your name, email address and password and then let Thunderbird have a stab in the dark at getting it. If you are lucky, Thunderbird will give up quickly and allow you to input your own settings, otherwise you have to wait while it guesses wrong. It is often here that Thunderbird tries to insist on IMAP.
If you try "Manual Setup..." here, there will be no return from the path towards IMAP and inevitable misery. Click "Edit" and then change the drop down from IMAP to POP.
You can then use "Manual Setup..." and put everything right - or just change them in the window above. The incoming mail server will almost certainly need changing, and the port number for POP3 without security will usually be 110.