"Continue" button greyed out when installing .pkg
From man installer:

SYNOPSIS
 installer ... -package &lt;pathToPackage&gt; -target device

[...]

A device parameter for the target is any one of the following:

 1) Any of the values returned by -dominfo
 2) The device node entry.  Any entry of the form of /dev/disk*.  ex: /dev/disk2
 3) The disk identifier.  Any entry of the form of disk*.  ex: disk1s9
 4) The volume mount point.  Any entry of the form of /Volumes/Mountpoint.  ex: /Volumes/Untitled
 5) The volume UUID.  ex: 376C4046-083E-334F-AF08-62FAFBC4E352


So try sudo installer ... -target / instead.

If this fails there is also the option of using Pacifist to extract the content of the package and install it manually (or do whatever you like to do with it).