If you're having issues connecting to a unit out of the box I'd make sure you're trying to communicate to the correct default address. Out of the box our 1550 should be accessible via Telnet, Console, and SSH at the address 10.10.10.1. Our quick start guide for that unit includes a more detailed explanation of how to connect to the box for the first time to configure it.
1550 Quick Start Guide: NetVanta 1550 Series Quick Start Guide
Let me know if you have any other questions!