On my system, it's on the root directory of my 'C' drive.
Try clicking Start -> Run, type in the following and click "OK":
This will also bring up that folder. If this gives an error, then there's something wrong with the package you've downloaded.
Another likely place the temporary files could be extracted to is the temp folder. You can open this folder by clicking start -> run, type in "%temp%" and click "OK". Sort the content by "Modified" and if the extracted files end up here, it will be one of the last folders listed.
If you would like to try extracting the main package file manually, first install 7-zip (download link), which is a very good freeware archive extraction utility. Once 7-zip is installed, right-click on your downloaded Office installation package, go into the "7-zip" sub-menu, select "Extract Files..." and specify where you would like to extract the contents to.