I like to use
wp-autoblog. Since it's no longer available by the author, I zipped up a copy of it for you.
Unzip and ftp the entire folder to you wp-content/plugins folder.
Login to your admin and activate the plug-in.
Go to Options and writing and set the default category to what best fits your feeds.
Then go options and wp autoblog.
Under basic settings add the feed / feeds URL you want to use.
Under intellectual property settings set whether you want to display the full feed or excerpts. Select whether to attribute the original author or not.
Click save and then click run script now.
The plugin will periodically check to see if the feed / feeds you added have updated and automatically update your blog.