On the mac, edit /etc/exports and add a line like so:
"/Users/<username>/develop/" -network 10.211.55.0 -mask 255.255.255.0 -mapall=<username>
The network and mask parameters indicate what hosts we'll share with; in my case, this network is used by the parallels shared network adapter; yours may differ.
You may need to reload nfsd:
sudo nfsd update
And you can check to see that the mounts are indeed exported
showmount -e
On the Linux host, just install the nfs utilities and mount as you normally would. Something like:
mount -t nfs -o proto=tcp,port=2049 10.211.55.2:/Users/<username>/develop /mnt/develop
The IP in this case is the ip the guest sees for the host. On a linux box, I just use the who command to get this.
Note that if you get a "bad superblock" or other error here, you probably neglected to install an nfs client, and are a big dummy. On ubuntu, you would do an apt-get install nfs-common. Non-apt distributions will differ.