It's pretty easy to use within another module - I've recently added this with the new version of the Feedback Module and  here are the steps.

In the ascx file add the following at the top
<%@ Register TagPrefix="dnn" Assembly="DotNetNuke" Namespace="DotNetNuke.UI.WebControls"%>

Within your page itself, include the following code

 <tr id="trCaptcha1" runat="server">
  <td class="SubHead" align="center"><dnn:label id="plCaptcha" controlname="ctlCaptcha" runat="server" text="Captcha:"></dnn:label></td>
 </tr>
 <tr id="trCaptcha2" runat="server">
  <td  align="center"><dnn:captchacontrol  id="ctlCaptcha" captchawidth="130" captchaheight="40" cssclass="Normal" runat="server" errorstyle-cssclass="NormalRed"  /></td>
 </tr>

Then within your form's submit event do something like this
 If (useCaptcha And ctlCaptcha.IsValid) OrElse (Not useCaptcha) Then
             ... process your form's other variables here.....

There's something with compression turned on which is causing the image to not show up (in 4.4.1), but if you have compression turned off, the image does show up as expected.

Also, remember to add the relevant label tags in your .resx (resource) files......