1. Open Synaptic
2. Check the box next to linux-686-smp kernel
3. Click Apply
Do you mean that the SMP kernel is not loaded as the default kernel? That's a different situation (that many would say is not a problem).
As for Nvidia drivers not working with a dual-core processor in Edgy, I have not found anything saying that is the case... I'd be interested in seeing some threads or links on that.