To mount a remote NFS exported directory to your Boxee Box:
mount -o nolock ip_of_nfs_server:/tmp /data/foo/
(Forgetting the '-o nolock' causes it to hang and then return an error.)
You'll need to setup /etc/exports on your Linux box to make this work.
This document details setting up an NFS Server: http://nfs.sourceforge.net/nfs-howto/ar01s03.html
Please note that currently the Boxee Box cannot export its own filesystems as NFS. The instructions are provided for guidance if you want to setup a Linux box to share directories via NFS.