Do a search about this for more detailed info -- there are a few very indepth threads a couple pages down in the slack forum too. Use the search function to search within the slack and/or slack-install forums.
The 2.6 kernel included with slack is an extra, you have to upgrade to it after the initial install. To install slackware on an sata box, you need to either create your own bootdisk, or download one that runs a kernel with support on it.
Then you choose the option in the slackware installer "Use kernel from bootdisk".
Do some searching for more in depth info -- where to get a good 2.6 bootdisk, for example
--Shade