Cats do like scratching posts... but consistent with feline perversity they won't pay any attention to anything you actually provide as a scratching post. My cat luckily chose a 2*4 we have propping up a trellis in the garden, after 8 years he's nearly half way through it, heh.
The best I can suggest is put a post of wood and stake it into a part of the garden your cat spends a lot of time in or around, buying a special post from a pet store will likely be a waste of time and money. Some cats prefer a tree to anything else.
As for the carpet; get some orange peel and put it around the area where it's being scratched. Cats despise citrus smells and shouldn't go anywhere near it. You can also get orange scented sprays from some pet shops that do the same job, but after a few days of leaving orange peel there the cat will probably have forgotten about it and, hopefully, found a better scratching post outside.
There's no way to make a cat choose a specific scratching post, but you can keep reducing their options till they choose one that you're happy with.