Mount NTFS filesystem

[Jack Bates <q34yzh@nottheoilrig.com>]

In the Debian Installer Rescue mode, how do I mount an NTFS filesystem?

When I boot debian-7.5.0-amd64-i386-netinst.iso and try,
I get the following error:

$ mount /dev/sdb1 /mnt
mount: mounting /dev/sdb1 on /mnt failed: Invalid argument

I suspect mounting an NTFS filesystem has something to do with the 
ntfs-3g package, but I'm not sure how to install packages in the 
installer environment, or if it's already installed?

In Rescue mode, how do I check which packages are already installed?
How do I get a list of available packages?

Thanks!