An interface needs to have an IP address to be considered layer 3. I would think that you could apply an IP address to a vlan and enable ip helper on that VLAN interface. Have you tried that? Page 246 of the user guide has some information on DHCP relay, followed by an example configuration. [URL] 
I have a Cisco core, I am integrating Dell into my network because I have heard great things.  I need to set up DHCP Relays on my vlans (coming from a cisco background) and found that it has to be applied at the interface not the Vlan. I tried to apply it to my interfaces and I get "interface not in layer 3 mode". So this is my problem, am I to put all my ports into Layer 3 to get my relays added? This is on Dell S3148P switches.
DHCP Relays Dell S3100 Series Switch Issues