Hello,
the update looks for the .msi in the original place you installed it from.
If you used the full installer, you would need to have the unpacked files in the same location (i.e. desktop or same folder where the full installer is/was placed), but the patcher could be looking for the .msi in a temporary folder which is no longer there.
If you can’t remember the exact location or have problems regardless, you can uninstall Cubase Pro 8 from Programs and Features (the application only, e.g. Steinberg Cubase 8 64 bit), run the full installer again (it will only install Cubase Pro 8 leaving all the content untouched) and then run the 8.0.30 update.
sps, it looks like this is what you need to do.
For those who initially used the ISO image and have the same problem: mounting the image so that the .msi can be found by the installer in a virtual drive allows the installation to complete.
Kind regards,