I have installed this plug in in my blog, WP 2.7.1 and success.
NOTE: before use this plug in, I HAVE INSTALLED "Contact Form 7" Plug in.
Better if you familiar with "Contact Form 7" plug in.
1. Upload this plug in at your
2. Install from your wp-admin area
3. Open your admin area of "Contact Form 7" plug in, because "Really Simple CAPTCHA" is INTEGRATED with "Contact Form 7".
4. At your form of "Contact Form 7" plug in, add new Tag of CAPTCHA to it.
5. See to below of the form of Really Simple CAPTCHA. U will get two codes that you must copy to your "Contact Form 7" form. And Save it.
6. After that, copy your code from "Contact Form 7" plug in to your page or post. Publish. And you will have a nice form with Captcha.
Hope it's useful