Hi,
I have Joomla! 1.6.3 Stable [ Onward ] 18-Apr-2011 23:00 GMT with installed JoomSEF version 4.0.4. I bought the JoomSEF plugin, but don't know, how to register in Joomla Admin.

I tried "Online Upgrade to Paid Version" in Upgrade tab, but get this message:

Provided Download ID is not valid for the requested file: com_joomsef3.zip. Please, make sure you are using valid Download ID for this product. (com_joomsef4.zip)

So I downloaded the com_joomsef4.zip from the site, but when I'm trying to install it (via "Upload Package File" in Upgrade tab) I got this error message:

Fatal error: Call to a member function children() on a non-object in /data/www/www_fotobanky_cz/tmp/install_4e41ab876511b/upgrade/4.0.9.php on line 77

Can you help me? I still have the advertisement links on my site.

Hi,

Make sure that you have the correct download ID set on the Registration tab in JoomSEF's Configuration. You should get the download ID in the e-mail after your purchase.
We will also check if there's any problem with our upgrade server.

I checked the download ID several times...

I have still the problem described above. Can you please look at it? Or I just throw away my money?

Just looking at it again and again and maybe the error is in versions? Check my first message:

I have Joomla! 1.6.3 Stable [ Onward ] 18-Apr-2011 23:00 GMT with installed JoomSEF version 4.0.4
...
Provided Download ID is not valid for the requested file: com_joomsef3.zip. Please, make sure you are using valid Download ID for this product. (com_joomsef3.zip)

Why there is once com_joomsef3.zip and then com_joomsef3.zip??

The fatal error in the first post is that you have installed joomsef sef_ext plugin without <files> section. Do you have installed some extra joomsef plugins?