su2kt3_05 folder IS the plug in . download it and install it in
c drive > program files > google > google sketch up > plugins
If you have already attempted the Install it would have been saved into the 'temporary internet files' as that is the deafault . You need to delete your temporary internet files first before you attempt it again - Tools > Internet options > delete temporary internet files
If you can't work that out you wont have much luck using Kerk really