I just had exactly the same problem – I had an old WordPress 2.2 export and was getting the same error.
I finally worked out how to solve it by doing an export of the ‘Hello World’ default post from a new installation of WP on my new server and seeing if it had a wxr version number in it. Which it did.
If you paste the following line into your export file after the Language defining line in the <channel> section near the topyou should be good to go:
<wp:wxr_version>1.1</wp:wxr_version>
You may be able to put it elsewhere, but that’s where it appeared in my new export and where it worked for me.
Hope that helps.
