Don't know what the Pottery Barn one looks like, but this is a great free pattern for a cabled afghan: http://www.lionbrand.com/patterns/70071AD.html?noImages=0.
Most yarn companies have free patterns, and Lionbrand is good because you can write to them if there is a problem with the directions. Also Bernat, Patons and Coats & Clark are very good for free patterns for basic sweaters. Once you are ready for more complicated things, have a go at knittingpatterncentral.com or garn studios or knitty.com. A great place for interesting patterns to download (they aren't free) is patternfish.com. Amazing stuff, there.