I can get you close. I've done this for several clients over the years.
Look in: /ideal_responsive/modules/One_Page_Checkout/opc_summary.tpl
Here is the code for the checkbox:
Code:
<div class="terms_n_conditions">
<label for="accept_warning">
<input type="checkbox" name="accept_warning" id="accept_warning" value="Y" />
You must also agree to this.
</label>
</div>
To do this right you should include some JavaScript to make sure they checked the box.