Jason -
If the stack topology is verified, there are three layers at which you could be experiencing problems. I'll list the order in which I would attempt to resolve this.
First, I would verify my layer 1 connection. when you issue a 'sho port XX:XX confi no" does it show that the port is active, or ready? If the port is active and the speed and duplex is correct, move to the next step, If it is in a ready state, this must be corrected before communication can be established.
Second, are the proper VLAN's with correct tags created in the new stack? is the uplink port properly configured with the VLAN(s)? The VLAN's/ID's/Tagged/Untagged values must match on both sides of the link if this is a trunk. Issue a "show edp port all" and this will display if the stack sees any directly connected Extreme devices across the layer 2 topology. If the VLANS, Tags, and trunk configurations are correct, we can move on to layer 3.
Last- on the new stack, have you given the proper IP and netmask to the vlan? is the vlan active? (sho vlan NAME - be sure that the vlan has member ports and that the vlan is active. If you are trying to ping local devices on the same subnet, this should be all you need. If you are trying to ping remote devices, have you configured a default route on the new switch?
Good luck!
-Sean