PowerConnect 6248P, setting the a native vlan? (using swichport mode general?)

I understand this has been talked about here before but I still cant find a working solution and I don't fully understand how to use switchport mode general to achieve this (or if I'm completly wrong using mode general?).

I have a computer and a nortel voip phone. Computer accesses vlan 100, phone either vlan 510 or 511 (depending on whats set in dhcp options). 

In Cisco land we would configure like this

switchport mode trunk

switchport trunk native vlan 100

switchport trunk allowed vlan 100,510,511

This would allow the PC to grab a 100 vlan IP from the DHCP server (ip helper in vlan 100's interface), and the phones to move over to either the 510 or 511 vlan once they had recived the DHCP options from the 100 scope. 

How do I replicate this on the 6248p's (I understand trunk native isn't available on the 6200's)? From what I've read its along the lines of 

switchport mode general
switchport general allowed vlan add 100
switchport general allowed vlan add 510-511 tagged

But that doesn't seem to be working (the computer isnt getting DHCP). The vlan is working on the switch, switchport access vlan 100 on the same port gets the computer through to the DHCP server straight away. 

Thanks in advance for any help.