There have been quite a few posts related to issues installing the VirtualBox Extension Pack for both Windows and Linux hosts.
The error message given is very cryptic:
"Failed to install the Extension Pack" NS_ERROR_FAILURE (0x80004005)
In digging through the above posts I found tidbits of the solution that I was able to put together. I'm currently running a Ubuntu 10.10 x64, so here's how I solved the problem and installed the extension pack. Give the VBoxExtPackHelperApp execute permissions and then run the install from the command line.
- sudo chmod 744 /usr/lib/virtualbox/VBoxExtPackHelperApp
- sudo /usr/lib/virtualbox/VBoxManage extpack install Oracle_VM_VirtualBox_Extension_Pack-4.0.4-70112.vbox-extpack