If you want to set up SpamAssassin with the highly cool KMail, here's how to go about it.

Make sure you run the SpamAssassin daemon (spamd) instead of running the command spamassassin, then set up a filter which tells KMail to pipe all messages under the size of 250000 (250kb) through the program spamc. Make sure this filter is the first one processed (top of the list). Then, just under that, have a filter which puts all messages where the X-Spam-Flag header equals YES (you will need to enter X-Spam-Flag yourself, KMail doesn't have it configured) into a specific folder.

What this does is it pipes all messages under 250kb through SpamAssassin (the size restriction is to stop KMail from choking itself on the large file size), then interprets the extra headers that SpamAssassin puts in to see whether a message is spam or not. If it is spam, it is put into your special folder for "filing" to Trash.

HTH.