Here's how to use the plugin.
1. Copy the reCaptcha folder under plugins directory.
2. Go to Preferences - Plugins in your blog dashboard.
3. You should see reCaptcha plugin listed.  Open reCaptcha plugin settings.
4. Input public and private key pair you acquired from reCaptcha.net for the blog.
5. Go to Blog Settings - Comment.
6. Choose reCaptcha as the blog's captcha provider and Save.

Users who are using the template set from MT 4.0x and MT 4.1x should follow the step 7 and 8 and skip the step 9.
Users who are using the template set from MT 4.2 should skip the step 7 and 8 and go to step 9.
NOTE that it is the version of the template that matters, not the version of the product you are using.  For example, you should follow the step 7 and step 8 and skip step 9 if you are using MT 4.2x that was upgraded from older versions, and have not refreshed your templates.

7. [MT 4.0x/4.1x users only] Edit Comment Form template, so it renders MTCaptchaFields unconditionally.  In practice, this means replacing these lines
    <MTIfNonEmpty tag="MTCaptchaFields">
    <MTIfCommentsAccepted><MTIfRegistrationAllowed><MTElse><$MTCaptchaFields$></MTIfRegistrationAllowed></MTIfCommentsAccepted>
    <div id="comments-open-captcha">
    </div>
    </MTIfNonEmpty>
to these lines:
    <MTIfNonEmpty tag="MTCaptchaFields">
    <div id="comments-open-captcha">
    <$MTCaptchaFields$>
    </div>
    </MTIfNonEmpty>
8. [MT 4.0x/4.1x users only] Edit Javascipt index template, to remove these lines of code which calls delayShowCaptcha:
    <MTIfNonEmpty tag="MTCaptchaFields">
    captcha_timer = setInterval('delayShowCaptcha()', 1000);
    </MTIfNonEmpty>

   If your install does not have these lines in JavaScript index template, these lines should be in GlobalJavaScript, which is a global system template.

9. [MT 4.2 users only] Edit Comments template module so it renders MTCaptchaFields by default.  In practice, this means you replace the line below
    <div id="comments-open-captcha"></div>
to this
    <div id="comments-open-captcha"><$MTCaptchaFields$></div>

10. Rebuild index and individual entry archives.