Solution for Google searchers:
Probably because your expansion disc has a newer installer. The Diablo II Gold version has the same issue.
Originally Posted by Driger
The Problem is that the CD contains permissions that wont allow your user to access the relevant files.
To fix this, you mus force your own user as owner of the files. This can be done (in a Terminal) with:
Assuming that the cd is unmounted. Then it should be possible to execute the installer normally.
sudo mount -o uid=$USER /media/cdrom
Note: To change the discs, you have to unmount the current one with
and mount the next one just like before (for the multi disk Gold version install).
sudo umount /media/cdrom