By stating "again", I assume you had Snow Leopard installed at some previous time. Apple currently posts: You can hold down the Option (⌥) or Alt at start up to boot to the Startup Manager, which allows you to choose other available startup disks or volumes. In this case you should select the optical drive containing the Snow Leopard DVD. Another option would be to hold down the C key at startup.
In your question, you did not state the exact model of your iMac or the version of Snow Leopard, therefore there is no guarantee the version of Snow Leopard is compatible with your iMac. For example, if the iMac is a 27-inch, Mid 2010 model (which came pre-installed with Snow Leopard OS X 10.6.3) and the version of Snow Leopard is OS X 10.6.0, then I would assume this version would not be usable.
If you do get Snow Leopard installed, then you should install the Mac OS X 10.6.8 Update Combo v1.1. You may have problems downloading using the iMac with Snow Leopard installed. You probably will want to download to a USB flash drive using Ubuntu on the iMac or your laptop.