Help setting up network with Airport Express (2009)

Hello

I just purchased my new Airport Extreme (draft-n) and setup was relatively easy when building a simple network. However, after discovering that the Extreme cannot cover the whole house, I've been wanting to use my old Asus WL-530g router as a base and the Extreme to extend the network, or vice versa.

I've tried using the Extreme as the extender to the Asus's network but it gives me this message: "You cannot extend this network"

What can be the problem?
Are there any alternative ways to extend a network?
Can I use the Asus itself to extend the Airport network?

 

I think that would work (I'll try it), but my 2nd router is an Asus, not an Express (I'm sorry the title says Express, I meant Extreme)