cgashlin01,
SIP Registration should only happen one way: from client to registrar. From your picture I would assume that 192.168.0.6 is the IP address of the Asterisk Server.
Have you performed a Wireshark Capture to see what is happening when the 5000 tries to register with the Asterisk Server?
I also went through the document and recreated the Asterisk side for the Trunking and have started creating a document for the 5000 side that I will post. When I do go over it and make sure it is what you have and if something needs to be changed on it.
Name: mitel-a
PEER
username=mitel
secret=Passw0rd1
host= 192.168.1.121
context=from-internal
nat=never <--Only if you are on the same network
dtmfmode=rfc2833
type=peer
allow=all
port=5060
User Context: from-mitel-a
USER:
username=mitel
secret=Passw0rd1
type=peer
reinvite=yes
port=5060
insecure=very
nat=never
host=192.168.1.121
fromdomain= 192.168.1.121
dtmfmode=rfc2833
context=from-trunk
allow=all
Thanks,
TE