Just insert the <iframe> ... </iframe> section from the client API:
<iframe src="
http://api.recaptcha.net/noscript?k=$publickey"
height="300" width="500" frameborder="0"></iframe><br>
<textarea name="recaptcha_challenge_field" rows="3" cols="40">
</textarea>
<input type="hidden" name="recaptcha_response_field"
value="manual_challenge">