There are 2 ways:
1) Debian has SMP-enabled kernel-images in the repositories, use Synaptic or aptitude to locate and install a version you want to use.
2) Install Debian as before, but make sure to include the kernel-sources. You may want to get a fresh kernel from
http://www.kernel.org, but the sources for the running kernel will do just fine. Now you have to configure the kernel to enable SMP support. A good guide for building your own kernel is here:
link!