Its a bit hard to tell exactly how the receiver and mixer are wired but it looks generally correct (and very neat!). So you get equal speed forwards and reverse on both motors? Is that the case when you move either the throttle or the steering on the transmitter? If the mixer leads are plugged in to the receiver throttle and steering channels, then you *should* be getting full control.

Maybe the issue is with transmitter - any info on that? At this stage, it could be that you are not getting any signal from the steering, just the throttle.