You'll need an IP address on the switch to be able to telnet through the management connection. The Link-Local IP would be flashing in the LCD window if your switch had 15.7 already installed (and one wasn't received from DHCP). The numbers in the stacking LCD would be illuminated in green if there was information to display.
You can't configure/enable stacking from within Chalet, so you'll need to connect to the switch using console so that you can configure the mgmt IP address.
How to console into Extreme and Enterasys switches
Once in, you can follow steps in the How-To article Brandon posted below.
If you're just using a laptop to connect to the switch, you won't need a default gateway, but you may need one at some point.
edit: Brandon created the mgmt interface article.