Is your "external" network in the admin site eth0 and "internal" eth1?
Has your external NIC got an IP address from your broadband DHCP server (if that's how you have it set up)?
Are both NICs (assuming you have 2) on different LAN segments and cannot talk to each other?
Can you browse the Internet from your LMCE?
b3 seems to be particularly sensitive to your NIC setup - I have had this problem several times, and invariably it has been because my external NIC was set to eth1 and internal to eth2. As soon as I worked out how to correct this, the software popped up in the screen and I could download/install it in seconds.,..